[pkg/ottl] Add support for scaling values (#33246)

**Description:**

Adds a `Scale` function to the OTTL package. This function can be
applied to `int`/`double` values, as well as metrics of the following
types:

- Sum
- Gauge
- Histogram

**Link to tracking Issue:** #16214

**Testing:** Added Unit and E2E tests in the OTTL package. Tested
manually in a sample environment with the following example
configuration:

```
receivers:
  otlp:
    protocols:
      grpc:
        endpoint: 0.0.0.0:4317

processors:
  transform:
    error_mode: ignore
    metric_statements:
      - context: metric
        statements:
          - set(data_points, Scale(data_points, 10.0))

exporters:
  debug:
    verbosity: detailed
  otlphttp:
    endpoint: "######"
    headers:
      Authorization: "#####"

service:
  pipelines:
    metrics:
      receivers: [otlp]
      exporters: [otlphttp, debug]
      processors: [transform]
```

**Documentation:** Added documentation in the `README` describing all
functions in the `ottl` package

---------

Signed-off-by: Florian Bacher <florian.bacher@dynatrace.com>
Co-authored-by: Evan Bradley <11745660+evan-bradley@users.noreply.github.com>
Co-authored-by: Tyler Helmuth <12352919+TylerHelmuth@users.noreply.github.com>

.chloggen/add-scale-function.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: processor/transform
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add `scale_metric` function that scales all data points in a metric.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [16214]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

processor/transformprocessor/README.md

@@ -212,6 +212,7 @@ In addition to OTTL functions, the processor defines its own functions to help w
 - [convert_summary_count_val_to_sum](#convert_summary_count_val_to_sum)
 - [convert_summary_sum_val_to_sum](#convert_summary_sum_val_to_sum)
 - [copy_metric](#copy_metric)
+- [scale_metric](#scale_metric)
 
 ### convert_sum_to_gauge
 
@@ -347,6 +348,21 @@ Examples:
 
 - `copy_metric(desc="new desc") where description == "old desc"`
 
+### scale_metric
+
+`scale_metric(factor, Optional[unit])`
+
+The `scale_metric` function multiplies the values in the data points in the metric by the float value `factor`.
+If the optional string `unit` is provided, the metric's unit will be set to this value.
+The supported data types are:
+
+Supported metric types are `Gauge`, `Sum`, `Histogram`, and `Summary`.
+
+Examples:
+
+- `scale_metric(0.1)`: Scale the metric by a factor of `0.1`. The unit of the metric will not be modified.
+- `scale_metric(10.0, "kWh")`: Scale the metric by a factor of `10.0` and sets the unit to `kWh`.
+
 ## Examples
 
 ### Perform transformation if field does not exist

processor/transformprocessor/internal/metrics/func_scale.go

@@ -0,0 +1,130 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package metrics // import "github.com/open-telemetry/opentelemetry-collector-contrib/processor/transformprocessor/internal/metrics"
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"go.opentelemetry.io/collector/pdata/pmetric"
+
+	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl"
+	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl/contexts/ottlmetric"
+)
+
+type ScaleArguments struct {
+	Multiplier float64
+	Unit       ottl.Optional[ottl.StringGetter[ottlmetric.TransformContext]]
+}
+
+func newScaleMetricFactory() ottl.Factory[ottlmetric.TransformContext] {
+	return ottl.NewFactory("scale_metric", &ScaleArguments{}, createScaleFunction)
+}
+
+func createScaleFunction(_ ottl.FunctionContext, oArgs ottl.Arguments) (ottl.ExprFunc[ottlmetric.TransformContext], error) {
+	args, ok := oArgs.(*ScaleArguments)
+
+	if !ok {
+		return nil, fmt.Errorf("ScaleFactory args must be of type *ScaleArguments[K]")
+	}
+
+	return Scale(*args)
+}
+
+func Scale(args ScaleArguments) (ottl.ExprFunc[ottlmetric.TransformContext], error) {
+	return func(ctx context.Context, tCtx ottlmetric.TransformContext) (any, error) {
+		metric := tCtx.GetMetric()
+
+		var unit *string
+		if !args.Unit.IsEmpty() {
+			u, err := args.Unit.Get().Get(ctx, tCtx)
+			if err != nil {
+				return nil, fmt.Errorf("could not get unit from ScaleArguments: %w", err)
+			}
+			unit = &u
+		}
+
+		switch metric.Type() {
+		case pmetric.MetricTypeGauge:
+			scaleMetric(metric.Gauge().DataPoints(), args.Multiplier)
+		case pmetric.MetricTypeHistogram:
+			scaleHistogram(metric.Histogram().DataPoints(), args.Multiplier)
+		case pmetric.MetricTypeSummary:
+			scaleSummarySlice(metric.Summary().DataPoints(), args.Multiplier)
+		case pmetric.MetricTypeSum:
+			scaleMetric(metric.Sum().DataPoints(), args.Multiplier)
+		case pmetric.MetricTypeExponentialHistogram:
+			return nil, errors.New("exponential histograms are not supported by the 'scale_metric' function")
+		default:
+			return nil, fmt.Errorf("unsupported metric type: '%v'", metric.Type())
+		}
+		if unit != nil {
+			metric.SetUnit(*unit)
+		}
+
+		return nil, nil
+	}, nil
+}
+
+func scaleExemplar(ex *pmetric.Exemplar, multiplier float64) {
+	switch ex.ValueType() {
+	case pmetric.ExemplarValueTypeInt:
+		ex.SetIntValue(int64(float64(ex.IntValue()) * multiplier))
+	case pmetric.ExemplarValueTypeDouble:
+		ex.SetDoubleValue(ex.DoubleValue() * multiplier)
+	}
+}
+
+func scaleSummarySlice(values pmetric.SummaryDataPointSlice, multiplier float64) {
+	for i := 0; i < values.Len(); i++ {
+		dp := values.At(i)
+
+		dp.SetSum(dp.Sum() * multiplier)
+
+		for i := 0; i < dp.QuantileValues().Len(); i++ {
+			qv := dp.QuantileValues().At(i)
+			qv.SetValue(qv.Value() * multiplier)
+		}
+	}
+}
+
+func scaleHistogram(datapoints pmetric.HistogramDataPointSlice, multiplier float64) {
+	for i := 0; i < datapoints.Len(); i++ {
+		dp := datapoints.At(i)
+
+		if dp.HasSum() {
+			dp.SetSum(dp.Sum() * multiplier)
+		}
+		if dp.HasMin() {
+			dp.SetMin(dp.Min() * multiplier)
+		}
+		if dp.HasMax() {
+			dp.SetMax(dp.Max() * multiplier)
+		}
+
+		for bounds, bi := dp.ExplicitBounds(), 0; bi < bounds.Len(); bi++ {
+			bounds.SetAt(bi, bounds.At(bi)*multiplier)
+		}
+
+		for exemplars, ei := dp.Exemplars(), 0; ei < exemplars.Len(); ei++ {
+			exemplar := exemplars.At(ei)
+			scaleExemplar(&exemplar, multiplier)
+		}
+	}
+}
+
+func scaleMetric(points pmetric.NumberDataPointSlice, multiplier float64) {
+	for i := 0; i < points.Len(); i++ {
+		dp := points.At(i)
+		switch dp.ValueType() {
+		case pmetric.NumberDataPointValueTypeInt:
+			dp.SetIntValue(int64(float64(dp.IntValue()) * multiplier))
+
+		case pmetric.NumberDataPointValueTypeDouble:
+			dp.SetDoubleValue(dp.DoubleValue() * multiplier)
+		default:
+		}
+	}
+}

processor/transformprocessor/internal/metrics/func_scale_test.go

@@ -0,0 +1,204 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package metrics
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"go.opentelemetry.io/collector/pdata/pcommon"
+	"go.opentelemetry.io/collector/pdata/pmetric"
+
+	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl"
+	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl/contexts/ottlmetric"
+)
+
+func TestScale(t *testing.T) {
+	type testCase struct {
+		name      string
+		args      ScaleArguments
+		valueFunc func() pmetric.Metric
+		wantFunc  func() pmetric.Metric
+		wantErr   bool
+	}
+	tests := []testCase{
+		{
+			name: "scale gauge float metric",
+			valueFunc: func() pmetric.Metric {
+				metric := pmetric.NewMetric()
+				metric.SetName("test-metric")
+				metric.SetEmptyGauge()
+				metric.Gauge().DataPoints().AppendEmpty().SetDoubleValue(10.0)
+
+				return metric
+			},
+			args: ScaleArguments{
+				Multiplier: 10.0,
+				Unit: ottl.NewTestingOptional[ottl.StringGetter[ottlmetric.TransformContext]](ottl.StandardStringGetter[ottlmetric.TransformContext]{
+					Getter: func(_ context.Context, _ ottlmetric.TransformContext) (any, error) {
+						return "kWh", nil
+					},
+				}),
+			},
+			wantFunc: func() pmetric.Metric {
+				metric := pmetric.NewMetric()
+				metric.SetName("test-metric")
+				metric.SetEmptyGauge()
+				metric.SetUnit("kWh")
+				metric.Gauge().DataPoints().AppendEmpty().SetDoubleValue(100.0)
+
+				return metric
+			},
+			wantErr: false,
+		},
+		{
+			name: "scale gauge int metric",
+			valueFunc: func() pmetric.Metric {
+				metric := pmetric.NewMetric()
+				metric.SetName("test-metric")
+				metric.SetEmptyGauge()
+				metric.Gauge().DataPoints().AppendEmpty().SetIntValue(10)
+
+				return metric
+			},
+			args: ScaleArguments{
+				Multiplier: 10.0,
+			},
+			wantFunc: func() pmetric.Metric {
+				metric := pmetric.NewMetric()
+				metric.SetName("test-metric")
+				metric.SetEmptyGauge()
+				metric.Gauge().DataPoints().AppendEmpty().SetIntValue(100.0)
+
+				return metric
+			},
+			wantErr: false,
+		},
+		{
+			name: "scale sum metric",
+			valueFunc: func() pmetric.Metric {
+				metric := pmetric.NewMetric()
+				metric.SetName("test-metric")
+				metric.SetEmptySum()
+				metric.Sum().DataPoints().AppendEmpty().SetDoubleValue(10.0)
+
+				return metric
+			},
+			args: ScaleArguments{
+				Multiplier: 10.0,
+			},
+			wantFunc: func() pmetric.Metric {
+				metric := pmetric.NewMetric()
+				metric.SetName("test-metric")
+				metric.SetEmptySum()
+				metric.Sum().DataPoints().AppendEmpty().SetDoubleValue(100.0)
+
+				return metric
+			},
+			wantErr: false,
+		},
+		{
+			name: "scale histogram metric",
+			valueFunc: func() pmetric.Metric {
+				metric := getTestScalingHistogramMetric(1, 4, 1, 3, []float64{1, 10}, []uint64{1, 2}, []float64{1.0}, 1, 1)
+				return metric
+			},
+			args: ScaleArguments{
+				Multiplier: 10.0,
+			},
+			wantFunc: func() pmetric.Metric {
+				metric := getTestScalingHistogramMetric(1, 40, 10, 30, []float64{10, 100}, []uint64{1, 2}, []float64{10.0}, 1, 1)
+				return metric
+			},
+			wantErr: false,
+		},
+		{
+			name: "scale summary metric",
+			valueFunc: func() pmetric.Metric {
+				metric := pmetric.NewMetric()
+				dp := metric.SetEmptySummary().DataPoints().AppendEmpty()
+				dp.SetSum(10.0)
+				qv := dp.QuantileValues().AppendEmpty()
+				qv.SetValue(10.0)
+
+				return metric
+			},
+			args: ScaleArguments{
+				Multiplier: 10.0,
+			},
+			wantFunc: func() pmetric.Metric {
+				metric := pmetric.NewMetric()
+				dp := metric.SetEmptySummary().DataPoints().AppendEmpty()
+				dp.SetSum(100.0)
+				qv := dp.QuantileValues().AppendEmpty()
+				qv.SetValue(100.0)
+
+				return metric
+			},
+			wantErr: false,
+		},
+		{
+			name: "unsupported: exponential histogram metric",
+			valueFunc: func() pmetric.Metric {
+				metric := pmetric.NewMetric()
+				metric.SetEmptyExponentialHistogram()
+				return metric
+			},
+			args: ScaleArguments{
+				Multiplier: 10.0,
+			},
+			wantFunc: func() pmetric.Metric {
+				// value should not be modified
+				metric := pmetric.NewMetric()
+				metric.SetEmptyExponentialHistogram()
+				return metric
+			},
+			wantErr: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			target := ottlmetric.NewTransformContext(
+				tt.valueFunc(),
+				pmetric.NewMetricSlice(),
+				pcommon.NewInstrumentationScope(),
+				pcommon.NewResource(),
+				pmetric.NewScopeMetrics(),
+				pmetric.NewResourceMetrics(),
+			)
+
+			expressionFunc, _ := Scale(tt.args)
+			_, err := expressionFunc(context.Background(), target)
+
+			if tt.wantErr {
+				assert.Error(t, err)
+			} else {
+				assert.NoError(t, err)
+			}
+			assert.EqualValues(t, tt.wantFunc(), target.GetMetric())
+		})
+	}
+}
+
+func getTestScalingHistogramMetric(count uint64, sum, min, max float64, bounds []float64, bucketCounts []uint64, exemplars []float64, start, timestamp pcommon.Timestamp) pmetric.Metric {
+	metric := pmetric.NewMetric()
+	metric.SetName("test-metric")
+	metric.SetEmptyHistogram()
+	histogramDatapoint := metric.Histogram().DataPoints().AppendEmpty()
+	histogramDatapoint.SetCount(count)
+	histogramDatapoint.SetSum(sum)
+	histogramDatapoint.SetMin(min)
+	histogramDatapoint.SetMax(max)
+	histogramDatapoint.ExplicitBounds().FromRaw(bounds)
+	histogramDatapoint.BucketCounts().FromRaw(bucketCounts)
+	for i := 0; i < len(exemplars); i++ {
+		exemplar := histogramDatapoint.Exemplars().AppendEmpty()
+		exemplar.SetTimestamp(1)
+		exemplar.SetDoubleValue(exemplars[i])
+	}
+	histogramDatapoint.SetStartTimestamp(start)
+	histogramDatapoint.SetTimestamp(timestamp)
+	return metric
+}