feat: return metric with an explicit timestamp from the configured column

README.md

@@ -188,6 +188,9 @@ metrics:
     # Static metric value (optional). Useful in case we are interested in string data (key_labels) only. It's mutually
     # exclusive with `values` field.
     # static_value: 1
+    # Timestamp value (optional). Should point at the existing column containing valid timestamps to return a metric
+    # with an explicit timestamp.
+    # timestamp_value: CreatedAt
     query: |
       SELECT Market, max(UpdateTime) AS LastUpdateTime
       FROM MarketPrices

config/metric_config.go

@@ -22,6 +22,7 @@ type MetricConfig struct {
 
 	NoPreparedStatement bool     `yaml:"no_prepared_statement,omitempty"` // do not prepare statement
 	StaticValue         *float64 `yaml:"static_value,omitempty"`
+	TimestampValue      string   `yaml:"timestamp_value,omitempty"` // optional column name containing a valid timestamp value
 
 	valueType prometheus.ValueType // TypeString converted to prometheus.ValueType
 	query     *QueryConfig         // QueryConfig resolved from QueryRef or generated from Query

documentation/sql_exporter.yml

@@ -76,6 +76,9 @@ collectors:
         # Arbitrary key/value pair
           env: dev
           region: europe
+        # Optional timestamp_value to point at the existing timestamp column to return a metric with an explicit
+        # timestamp.
+        # timestamp_value: CreatedAt
         # This query returns exactly one value per row, in the `counter` column.
         values: [counter]
         query: |

metric.go

@@ -4,6 +4,7 @@ import (
 	"database/sql"
 	"fmt"
 	"sort"
+	"time"
 
 	"github.com/burningalchemist/sql_exporter/config"
 	"github.com/burningalchemist/sql_exporter/errors"
@@ -85,7 +86,15 @@ func (mf MetricFamily) Collect(row map[string]any, ch chan<- Metric) {
 		}
 		value := row[v].(sql.NullFloat64)
 		if value.Valid {
-			ch <- NewMetric(&mf, value.Float64, labelValues...)
+			metric := NewMetric(&mf, value.Float64, labelValues...)
+			if mf.config.TimestampValue == "" {
+				ch <- metric
+			} else {
+				ts := row[mf.config.TimestampValue].(sql.NullTime)
+				if ts.Valid {
+					ch <- NewMetricWithTimestamp(ts.Time, metric)
+				}
+			}
 		}
 	}
 	if mf.config.StaticValue != nil {
@@ -286,3 +295,18 @@ func NewInvalidMetric(err errors.WithContext) Metric {
 func (m invalidMetric) Desc() MetricDesc { return nil }
 
 func (m invalidMetric) Write(*dto.Metric) errors.WithContext { return m.err }
+
+type timestampedMetric struct {
+	Metric
+	t time.Time
+}
+
+func (m timestampedMetric) Write(pb *dto.Metric) errors.WithContext {
+	e := m.Metric.Write(pb)
+	pb.TimestampMs = proto.Int64(m.t.Unix()*1000 + int64(m.t.Nanosecond()/1000000))
+	return e
+}
+
+func NewMetricWithTimestamp(t time.Time, m Metric) Metric {
+	return timestampedMetric{Metric: m, t: t}
+}

query.go

@@ -30,6 +30,7 @@ type (
 const (
 	columnTypeKey   columnType = 1
 	columnTypeValue columnType = 2
+	columnTypeTime  columnType = 3
 )
 
 // NewQuery returns a new Query that will populate the given metric families.
@@ -49,6 +50,9 @@ func NewQuery(logContext string, qc *config.QueryConfig, metricFamilies ...*Metr
 				return nil, err
 			}
 		}
+		if err := setColumnType(logContext, mf.config.TimestampValue, columnTypeTime, columnTypes); err != nil {
+			return nil, err
+		}
 	}
 
 	q := Query{
@@ -153,6 +157,9 @@ func (q *Query) scanDest(rows *sql.Rows) ([]any, errors.WithContext) {
 		case columnTypeValue:
 			dest = append(dest, new(sql.NullFloat64))
 			have[column] = true
+		case columnTypeTime:
+			dest = append(dest, new(sql.NullTime))
+			have[column] = true
 		default:
 			if column == "" {
 				klog.Infof("[%s] Unnamed column %d returned by query", q.logContext, i)
@@ -199,6 +206,11 @@ func (q *Query) scanRow(rows *sql.Rows, dest []any) (map[string]any, errors.With
 				klog.V(3).Infof("[%s] Key column %q is NULL", q.logContext, column)
 			}
 			result[column] = *dest[i].(*sql.NullString)
+		case columnTypeTime:
+			if !dest[i].(*sql.NullTime).Valid {
+				klog.V(3).Infof("[%s] Time column %q is invalid or NULL", q.logContext, column)
+			}
+			result[column] = *dest[i].(*sql.NullTime)
 		case columnTypeValue:
 			if !dest[i].(*sql.NullFloat64).Valid {
 				klog.V(3).Infof("[%s] Value column %q is NULL", q.logContext, column)