Metric SDK: Do not export non-observed attribute sets for async instruments (#4290)

* drop non-observed attribute sets

* fix test comment

* add documentation for async callbacks dropping unobserved attributes

sdk/metric/internal/aggregate/sum.go

@@ -255,10 +255,12 @@ type precomputedDeltaSum[N int64 | float64] struct {
 // collection cycle, and the unfiltered-sum is kept for the next collection
 // cycle.
 func (s *precomputedDeltaSum[N]) Aggregation() metricdata.Aggregation {
+	newReported := make(map[attribute.Set]N)
 	s.Lock()
 	defer s.Unlock()
 
 	if len(s.values) == 0 {
+		s.reported = newReported
 		return nil
 	}
 
@@ -277,16 +279,12 @@ func (s *precomputedDeltaSum[N]) Aggregation() metricdata.Aggregation {
 			Time:       t,
 			Value:      delta,
 		})
-		if delta != 0 {
-			s.reported[attr] = v
-		}
-		value.filtered = N(0)
-		s.values[attr] = value
-		// TODO (#3006): This will use an unbounded amount of memory if there
-		// are unbounded number of attribute sets being aggregated. Attribute
-		// sets that become "stale" need to be forgotten so this will not
-		// overload the system.
+		newReported[attr] = v
+		// Unused attribute sets do not report.
+		delete(s.values, attr)
 	}
+	// Unused attribute sets are forgotten.
+	s.reported = newReported
 	// The delta collection cycle resets.
 	s.start = t
 	return out
@@ -349,12 +347,8 @@ func (s *precomputedCumulativeSum[N]) Aggregation() metricdata.Aggregation {
 			Time:       t,
 			Value:      value.measured + value.filtered,
 		})
-		value.filtered = N(0)
-		s.values[attr] = value
-		// TODO (#3006): This will use an unbounded amount of memory if there
-		// are unbounded number of attribute sets being aggregated. Attribute
-		// sets that become "stale" need to be forgotten so this will not
-		// overload the system.
+		// Unused attribute sets do not report.
+		delete(s.values, attr)
 	}
 	return out
 }

sdk/metric/internal/aggregate/sum_test.go

@@ -180,10 +180,9 @@ func TestPreComputedDeltaSum(t *testing.T) {
 	opt := metricdatatest.IgnoreTimestamp()
 	metricdatatest.AssertAggregationsEqual(t, want, got, opt)
 
-	// Delta values should zero.
+	// No observation means no metric data
 	got = agg.Aggregation()
-	want.DataPoints = []metricdata.DataPoint[int64]{point[int64](attrs, 0)}
-	metricdatatest.AssertAggregationsEqual(t, want, got, opt)
+	metricdatatest.AssertAggregationsEqual(t, nil, got, opt)
 
 	agg.(precomputeAggregator[int64]).aggregateFiltered(1, attrs)
 	got = agg.Aggregation()
@@ -193,13 +192,8 @@ func TestPreComputedDeltaSum(t *testing.T) {
 
 	// Filtered values should not persist.
 	got = agg.Aggregation()
-	// measured(+): 1, previous(-): 2, filtered(+): 0
-	want.DataPoints = []metricdata.DataPoint[int64]{point[int64](attrs, -1)}
-	metricdatatest.AssertAggregationsEqual(t, want, got, opt)
-	got = agg.Aggregation()
-	// measured(+): 1, previous(-): 1, filtered(+): 0
-	want.DataPoints = []metricdata.DataPoint[int64]{point[int64](attrs, 0)}
-	metricdatatest.AssertAggregationsEqual(t, want, got, opt)
+	// No observation means no metric data
+	metricdatatest.AssertAggregationsEqual(t, nil, got, opt)
 
 	// Override set value.
 	agg.Aggregate(2, attrs)
@@ -208,8 +202,8 @@ func TestPreComputedDeltaSum(t *testing.T) {
 	agg.(precomputeAggregator[int64]).aggregateFiltered(3, attrs)
 	agg.(precomputeAggregator[int64]).aggregateFiltered(10, attrs)
 	got = agg.Aggregation()
-	// measured(+): 5, previous(-): 1, filtered(+): 13
-	want.DataPoints = []metricdata.DataPoint[int64]{point[int64](attrs, 17)}
+	// measured(+): 5, previous(-): 0, filtered(+): 13
+	want.DataPoints = []metricdata.DataPoint[int64]{point[int64](attrs, 18)}
 	metricdatatest.AssertAggregationsEqual(t, want, got, opt)
 
 	// Filtered values should not persist.
@@ -251,19 +245,18 @@ func TestPreComputedCumulativeSum(t *testing.T) {
 	opt := metricdatatest.IgnoreTimestamp()
 	metricdatatest.AssertAggregationsEqual(t, want, got, opt)
 
-	// Cumulative values should persist.
+	// Cumulative values should not persist.
 	got = agg.Aggregation()
-	metricdatatest.AssertAggregationsEqual(t, want, got, opt)
+	metricdatatest.AssertAggregationsEqual(t, nil, got, opt)
 
 	agg.(precomputeAggregator[int64]).aggregateFiltered(1, attrs)
 	got = agg.Aggregation()
-	want.DataPoints = []metricdata.DataPoint[int64]{point[int64](attrs, 2)}
+	want.DataPoints = []metricdata.DataPoint[int64]{point[int64](attrs, 1)}
 	metricdatatest.AssertAggregationsEqual(t, want, got, opt)
 
 	// Filtered values should not persist.
 	got = agg.Aggregation()
-	want.DataPoints = []metricdata.DataPoint[int64]{point[int64](attrs, 1)}
-	metricdatatest.AssertAggregationsEqual(t, want, got, opt)
+	metricdatatest.AssertAggregationsEqual(t, nil, got, opt)
 
 	// Override set value.
 	agg.Aggregate(5, attrs)
@@ -276,8 +269,7 @@ func TestPreComputedCumulativeSum(t *testing.T) {
 
 	// Filtered values should not persist.
 	got = agg.Aggregation()
-	want.DataPoints = []metricdata.DataPoint[int64]{point[int64](attrs, 5)}
-	metricdatatest.AssertAggregationsEqual(t, want, got, opt)
+	metricdatatest.AssertAggregationsEqual(t, nil, got, opt)
 
 	// Order should not affect measure.
 	// Filtered should add.
@@ -287,9 +279,6 @@ func TestPreComputedCumulativeSum(t *testing.T) {
 	got = agg.Aggregation()
 	want.DataPoints = []metricdata.DataPoint[int64]{point[int64](attrs, 20)}
 	metricdatatest.AssertAggregationsEqual(t, want, got, opt)
-	got = agg.Aggregation()
-	want.DataPoints = []metricdata.DataPoint[int64]{point[int64](attrs, 7)}
-	metricdatatest.AssertAggregationsEqual(t, want, got, opt)
 }
 
 func TestEmptySumNilAggregation(t *testing.T) {

sdk/metric/meter.go

@@ -107,6 +107,7 @@ func (m *meter) Int64Histogram(name string, options ...metric.Int64HistogramOpti
 // Int64ObservableCounter returns a new instrument identified by name and
 // configured with options. The instrument is used to asynchronously record
 // increasing int64 measurements once per a measurement collection cycle.
+// Only the measurements recorded during the collection cycle are exported.
 func (m *meter) Int64ObservableCounter(name string, options ...metric.Int64ObservableCounterOption) (metric.Int64ObservableCounter, error) {
 	cfg := metric.NewInt64ObservableCounterConfig(options...)
 	const kind = InstrumentKindObservableCounter
@@ -121,7 +122,8 @@ func (m *meter) Int64ObservableCounter(name string, options ...metric.Int64Obser
 
 // Int64ObservableUpDownCounter returns a new instrument identified by name and
 // configured with options. The instrument is used to asynchronously record
-// int64 measurements once per a measurement collection cycle.
+// int64 measurements once per a measurement collection cycle. Only the
+// measurements recorded during the collection cycle are exported.
 func (m *meter) Int64ObservableUpDownCounter(name string, options ...metric.Int64ObservableUpDownCounterOption) (metric.Int64ObservableUpDownCounter, error) {
 	cfg := metric.NewInt64ObservableUpDownCounterConfig(options...)
 	const kind = InstrumentKindObservableUpDownCounter
@@ -137,6 +139,7 @@ func (m *meter) Int64ObservableUpDownCounter(name string, options ...metric.Int6
 // Int64ObservableGauge returns a new instrument identified by name and
 // configured with options. The instrument is used to asynchronously record
 // instantaneous int64 measurements once per a measurement collection cycle.
+// Only the measurements recorded during the collection cycle are exported.
 func (m *meter) Int64ObservableGauge(name string, options ...metric.Int64ObservableGaugeOption) (metric.Int64ObservableGauge, error) {
 	cfg := metric.NewInt64ObservableGaugeConfig(options...)
 	const kind = InstrumentKindObservableGauge
@@ -194,6 +197,7 @@ func (m *meter) Float64Histogram(name string, options ...metric.Float64Histogram
 // Float64ObservableCounter returns a new instrument identified by name and
 // configured with options. The instrument is used to asynchronously record
 // increasing float64 measurements once per a measurement collection cycle.
+// Only the measurements recorded during the collection cycle are exported.
 func (m *meter) Float64ObservableCounter(name string, options ...metric.Float64ObservableCounterOption) (metric.Float64ObservableCounter, error) {
 	cfg := metric.NewFloat64ObservableCounterConfig(options...)
 	const kind = InstrumentKindObservableCounter
@@ -208,7 +212,8 @@ func (m *meter) Float64ObservableCounter(name string, options ...metric.Float64O
 
 // Float64ObservableUpDownCounter returns a new instrument identified by name
 // and configured with options. The instrument is used to asynchronously record
-// float64 measurements once per a measurement collection cycle.
+// float64 measurements once per a measurement collection cycle. Only the
+// measurements recorded during the collection cycle are exported.
 func (m *meter) Float64ObservableUpDownCounter(name string, options ...metric.Float64ObservableUpDownCounterOption) (metric.Float64ObservableUpDownCounter, error) {
 	cfg := metric.NewFloat64ObservableUpDownCounterConfig(options...)
 	const kind = InstrumentKindObservableUpDownCounter
@@ -224,6 +229,7 @@ func (m *meter) Float64ObservableUpDownCounter(name string, options ...metric.Fl
 // Float64ObservableGauge returns a new instrument identified by name and
 // configured with options. The instrument is used to asynchronously record
 // instantaneous float64 measurements once per a measurement collection cycle.
+// Only the measurements recorded during the collection cycle are exported.
 func (m *meter) Float64ObservableGauge(name string, options ...metric.Float64ObservableGaugeOption) (metric.Float64ObservableGauge, error) {
 	cfg := metric.NewFloat64ObservableGaugeConfig(options...)
 	const kind = InstrumentKindObservableGauge
@@ -272,6 +278,10 @@ func isAlphanumeric(c rune) bool {
 // Only instruments from this meter can be registered with f, an error is
 // returned if other instrument are provided.
 //
+// Only observations made in the callback will be exported. Unlike synchronous
+// instruments, asynchronous callbacks can "forget" attribute sets that are no
+// longer relevant by omitting the observation during the callback.
+//
 // The returned Registration can be used to unregister f.
 func (m *meter) RegisterCallback(f metric.Callback, insts ...metric.Observable) (metric.Registration, error) {
 	if len(insts) == 0 {

sdk/metric/meter_test.go

@@ -1687,8 +1687,7 @@ func TestObservableExample(t *testing.T) {
 			Temporality: temporality,
 			IsMonotonic: true,
 			DataPoints: []metricdata.DataPoint[int64]{
-				// Thread 1 remains at last measured value.
-				{Attributes: thread1, Value: 60},
+				// Thread 1 is no longer exported.
 				{Attributes: thread2, Value: 53},
 				{Attributes: thread3, Value: 5},
 			},
@@ -1762,8 +1761,7 @@ func TestObservableExample(t *testing.T) {
 			Temporality: temporality,
 			IsMonotonic: true,
 			DataPoints: []metricdata.DataPoint[int64]{
-				// Thread 1 remains at last measured value.
-				{Attributes: thread1, Value: 0},
+				// Thread 1 is no longer exported.
 				{Attributes: thread2, Value: 6},
 				{Attributes: thread3, Value: 5},
 			},