Use LastValue by default for ValueObserver instruments (#1165)

* Use LastValue by default for ValueObserver instruments

* Update test

* Update test with feedback

* Update fix

* Update changelog

* sum->scalar

* Tests pass

* Add a test

* Undo incorrect proto adds

* Restore origin protos

* Restore

* Upstream

* Add more tests

* Precommit

* Typo

* ore test feedback

CHANGELOG.md

@@ -45,6 +45,8 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Don't consider unset environment variable for resource detection to be an error. (#1170)
 - Rename `go.opentelemetry.io/otel/api/metric.ConfigureInstrument` to `NewInstrumentConfig` and
   `go.opentelemetry.io/otel/api/metric.ConfigureMeter` to `NewMeterConfig`.
+- ValueObserver instruments use LastValue aggregator by default. (#1165)
+- OTLP Metric exporter supports LastValue aggregation. (#1165)
 - Move the `go.opentelemetry.io/otel/api/unit` package to `go.opentelemetry.io/otel/unit`. (#1185)
 - Rename `Provider` to `MeterProvider` in the `go.opentelemetry.io/otel/api/metric` package. (#1190)
 - Rename `NoopProvider` to `NoopMeterProvider` in the `go.opentelemetry.io/otel/api/metric` package. (#1190)

exporters/otlp/README.md

@@ -48,7 +48,7 @@ func main() {
 	//   	),
 	//   )
 	tracerProvider := sdktrace.NewTracerProvider(sdktrace.WithBatcher(exporter))
-	pusher := push.New(simple.NewWithExactDistribution(), exporter)
+	pusher := push.New(simple.NewWithInexpensiveDistribution(), exporter)
 	pusher.Start()
 	metricProvider := pusher.MeterProvider()
 

exporters/otlp/internal/transform/metric.go

@@ -22,6 +22,7 @@ import (
 	"fmt"
 	"strings"
 	"sync"
+	"time"
 
 	commonpb "go.opentelemetry.io/otel/exporters/otlp/internal/opentelemetry-proto-gen/common/v1"
 	metricpb "go.opentelemetry.io/otel/exporters/otlp/internal/opentelemetry-proto-gen/metrics/v1"
@@ -40,6 +41,11 @@ var (
 	// aggregator is attempted.
 	ErrUnimplementedAgg = errors.New("unimplemented aggregator")
 
+	// ErrIncompatibleAgg is returned when
+	// aggregation.Kind implies an interface conversion that has
+	// failed
+	ErrIncompatibleAgg = errors.New("incompatible aggregation type")
+
 	// ErrUnknownValueType is returned when a transformation of an unknown value
 	// is attempted.
 	ErrUnknownValueType = errors.New("invalid value type")
@@ -60,6 +66,14 @@ type result struct {
 	Err                    error
 }
 
+// toNanos returns the number of nanoseconds since the UNIX epoch.
+func toNanos(t time.Time) uint64 {
+	if t.IsZero() {
+		return 0
+	}
+	return uint64(t.UnixNano())
+}
+
 // CheckpointSet transforms all records contained in a checkpoint into
 // batched OTLP ResourceMetrics.
 func CheckpointSet(ctx context.Context, exportSelector export.ExportKindSelector, cps export.CheckpointSet, numWorkers uint) ([]*metricpb.ResourceMetrics, error) {
@@ -231,27 +245,50 @@ func sink(ctx context.Context, in <-chan result) ([]*metricpb.ResourceMetrics, e
 	return rms, nil
 }
 
-// Record transforms a Record into an OTLP Metric. An ErrUnimplementedAgg
+// Record transforms a Record into an OTLP Metric. An ErrIncompatibleAgg
 // error is returned if the Record Aggregator is not supported.
 func Record(r export.Record) (*metricpb.Metric, error) {
-	switch a := r.Aggregation().(type) {
-	case aggregation.MinMaxSumCount:
-		return minMaxSumCount(r, a)
-	case aggregation.Sum:
-		return sum(r, a)
+	agg := r.Aggregation()
+	switch agg.Kind() {
+	case aggregation.MinMaxSumCountKind:
+		mmsc, ok := agg.(aggregation.MinMaxSumCount)
+		if !ok {
+			return nil, fmt.Errorf("%w: %T", ErrIncompatibleAgg, agg)
+		}
+		return minMaxSumCount(r, mmsc)
+
+	case aggregation.SumKind:
+		s, ok := agg.(aggregation.Sum)
+		if !ok {
+			return nil, fmt.Errorf("%w: %T", ErrIncompatibleAgg, agg)
+		}
+		sum, err := s.Sum()
+		if err != nil {
+			return nil, err
+		}
+		return scalar(r, sum, r.StartTime(), r.EndTime())
+
+	case aggregation.LastValueKind:
+		lv, ok := agg.(aggregation.LastValue)
+		if !ok {
+			return nil, fmt.Errorf("%w: %T", ErrIncompatibleAgg, agg)
+		}
+		value, tm, err := lv.LastValue()
+		if err != nil {
+			return nil, err
+		}
+		return scalar(r, value, time.Time{}, tm)
+
 	default:
-		return nil, fmt.Errorf("%w: %v", ErrUnimplementedAgg, a)
+		return nil, fmt.Errorf("%w: %T", ErrUnimplementedAgg, agg)
 	}
 }
 
-// sum transforms a Sum Aggregator into an OTLP Metric.
-func sum(record export.Record, a aggregation.Sum) (*metricpb.Metric, error) {
+// scalar transforms a Sum or LastValue Aggregator into an OTLP Metric.
+// For LastValue (Gauge), use start==time.Time{}.
+func scalar(record export.Record, num metric.Number, start, end time.Time) (*metricpb.Metric, error) {
 	desc := record.Descriptor()
 	labels := record.Labels()
-	sum, err := a.Sum()
-	if err != nil {
-		return nil, err
-	}
 
 	m := &metricpb.Metric{
 		MetricDescriptor: &metricpb.MetricDescriptor{
@@ -266,20 +303,20 @@ func sum(record export.Record, a aggregation.Sum) (*metricpb.Metric, error) {
 		m.MetricDescriptor.Type = metricpb.MetricDescriptor_INT64
 		m.Int64DataPoints = []*metricpb.Int64DataPoint{
 			{
-				Value:             sum.CoerceToInt64(n),
+				Value:             num.CoerceToInt64(n),
 				Labels:            stringKeyValues(labels.Iter()),
-				StartTimeUnixNano: uint64(record.StartTime().UnixNano()),
-				TimeUnixNano:      uint64(record.EndTime().UnixNano()),
+				StartTimeUnixNano: toNanos(start),
+				TimeUnixNano:      toNanos(end),
 			},
 		}
 	case metric.Float64NumberKind:
 		m.MetricDescriptor.Type = metricpb.MetricDescriptor_DOUBLE
 		m.DoubleDataPoints = []*metricpb.DoubleDataPoint{
 			{
-				Value:             sum.CoerceToFloat64(n),
+				Value:             num.CoerceToFloat64(n),
 				Labels:            stringKeyValues(labels.Iter()),
-				StartTimeUnixNano: uint64(record.StartTime().UnixNano()),
-				TimeUnixNano:      uint64(record.EndTime().UnixNano()),
+				StartTimeUnixNano: toNanos(start),
+				TimeUnixNano:      toNanos(end),
 			},
 		}
 	default:
@@ -339,8 +376,8 @@ func minMaxSumCount(record export.Record, a aggregation.MinMaxSumCount) (*metric
 						Value:      max.CoerceToFloat64(numKind),
 					},
 				},
-				StartTimeUnixNano: uint64(record.StartTime().UnixNano()),
-				TimeUnixNano:      uint64(record.EndTime().UnixNano()),
+				StartTimeUnixNano: toNanos(record.StartTime()),
+				TimeUnixNano:      toNanos(record.EndTime()),
 			},
 		},
 	}, nil