Add implementation of last-value aggregator (#3008)

* Add last-value aggregator

* Add test of last-value reset of unseen attrs

* Add benchmark

* Use generic DataPoint value

* Fix assertion_fail_test.go

* Fix tests

* Remove unused test increment values

sdk/metric/internal/aggregator_test.go

@@ -0,0 +1,154 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build go1.18
+// +build go1.18
+
+package internal // import "go.opentelemetry.io/otel/sdk/metric/internal"
+
+import (
+	"strconv"
+	"sync"
+	"testing"
+	"time"
+
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/sdk/metric/metricdata"
+	"go.opentelemetry.io/otel/sdk/metric/metricdata/metricdatatest"
+)
+
+const (
+	defaultGoroutines   = 5
+	defaultMeasurements = 30
+	defaultCycles       = 3
+)
+
+var (
+	alice = attribute.NewSet(attribute.String("user", "alice"), attribute.Bool("admin", true))
+	bob   = attribute.NewSet(attribute.String("user", "bob"), attribute.Bool("admin", false))
+	carol = attribute.NewSet(attribute.String("user", "carol"), attribute.Bool("admin", false))
+
+	monoIncr = setMap{alice: 1, bob: 10, carol: 2}
+
+	// Sat Jan 01 2000 00:00:00 GMT+0000.
+	staticTime    = time.Unix(946684800, 0)
+	staticNowFunc = func() time.Time { return staticTime }
+	// Pass to t.Cleanup to override the now function with staticNowFunc and
+	// revert once the test completes. E.g. t.Cleanup(mockTime(now)).
+	mockTime = func(orig func() time.Time) (cleanup func()) {
+		now = staticNowFunc
+		return func() { now = orig }
+	}
+)
+
+// setMap maps attribute sets to a number.
+type setMap map[attribute.Set]int
+
+// expectFunc is a function that returns an Aggregation of expected values for
+// a cycle that contains m measurements (total across all goroutines). Each
+// call advances the cycle.
+type expectFunc func(m int) metricdata.Aggregation
+
+// aggregatorTester runs an acceptance test on an Aggregator. It will ask an
+// Aggregator to aggregate a set of values as if they were real measurements
+// made MeasurementN number of times. This will be done in GoroutineN number
+// of different goroutines. After the Aggregator has been asked to aggregate
+// all these measurements, it is validated using a passed expecterFunc. This
+// set of operation is a signle cycle, and the the aggregatorTester will run
+// CycleN number of cycles.
+type aggregatorTester[N int64 | float64] struct {
+	// GoroutineN is the number of goroutines aggregatorTester will use to run
+	// the test with.
+	GoroutineN int
+	// MeasurementN is the number of measurements that are made each cycle a
+	// goroutine runs the test.
+	MeasurementN int
+	// CycleN is the number of times a goroutine will make a set of
+	// measurements.
+	CycleN int
+}
+
+func (at *aggregatorTester[N]) Run(a Aggregator[N], incr setMap, eFunc expectFunc) func(*testing.T) {
+	m := at.MeasurementN * at.GoroutineN
+	return func(t *testing.T) {
+		for i := 0; i < at.CycleN; i++ {
+			var wg sync.WaitGroup
+			wg.Add(at.GoroutineN)
+			for i := 0; i < at.GoroutineN; i++ {
+				go func() {
+					defer wg.Done()
+					for j := 0; j < at.MeasurementN; j++ {
+						for attrs, n := range incr {
+							a.Aggregate(N(n), attrs)
+						}
+					}
+				}()
+			}
+			wg.Wait()
+
+			metricdatatest.AssertAggregationsEqual(t, eFunc(m), a.Aggregation())
+		}
+	}
+}
+
+var bmarkResults metricdata.Aggregation
+
+func benchmarkAggregatorN[N int64 | float64](b *testing.B, factory func() Aggregator[N], count int) {
+	attrs := make([]attribute.Set, count)
+	for i := range attrs {
+		attrs[i] = attribute.NewSet(attribute.Int("value", i))
+	}
+
+	b.Run("Aggregate", func(b *testing.B) {
+		agg := factory()
+		b.ReportAllocs()
+		b.ResetTimer()
+
+		for n := 0; n < b.N; n++ {
+			for _, attr := range attrs {
+				agg.Aggregate(1, attr)
+			}
+		}
+		bmarkResults = agg.Aggregation()
+	})
+
+	b.Run("Aggregations", func(b *testing.B) {
+		aggs := make([]Aggregator[N], b.N)
+		for n := range aggs {
+			a := factory()
+			for _, attr := range attrs {
+				a.Aggregate(1, attr)
+			}
+			aggs[n] = a
+		}
+
+		b.ReportAllocs()
+		b.ResetTimer()
+
+		for n := 0; n < b.N; n++ {
+			bmarkResults = aggs[n].Aggregation()
+		}
+	})
+}
+
+func benchmarkAggregator[N int64 | float64](factory func() Aggregator[N]) func(*testing.B) {
+	counts := []int{1, 10, 100}
+	return func(b *testing.B) {
+		for _, n := range counts {
+			b.Run(strconv.Itoa(n), func(b *testing.B) {
+				benchmarkAggregatorN(b, factory, n)
+			})
+		}
+	}
+}

sdk/metric/internal/lastvalue.go

@@ -18,26 +18,64 @@
 package internal // import "go.opentelemetry.io/otel/sdk/metric/internal"
 
 import (
+	"sync"
+	"time"
+
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/sdk/metric/metricdata"
 )
 
+// now is used to return the current local time while allowing tests to
+// override the the default time.Now function.
+var now = time.Now
+
+// datapoint is timestamped measurement data.
+type datapoint[N int64 | float64] struct {
+	timestamp time.Time
+	value     N
+}
+
 // lastValue summarizes a set of measurements as the last one made.
 type lastValue[N int64 | float64] struct {
-	// TODO(#2971): implement.
+	sync.Mutex
+
+	values map[attribute.Set]datapoint[N]
 }
 
 // NewLastValue returns an Aggregator that summarizes a set of measurements as
 // the last one made.
 func NewLastValue[N int64 | float64]() Aggregator[N] {
-	return &lastValue[N]{}
+	return &lastValue[N]{values: make(map[attribute.Set]datapoint[N])}
 }
 
 func (s *lastValue[N]) Aggregate(value N, attr attribute.Set) {
-	// TODO(#2971): implement.
+	d := datapoint[N]{timestamp: now(), value: value}
+	s.Lock()
+	s.values[attr] = d
+	s.Unlock()
 }
 
 func (s *lastValue[N]) Aggregation() metricdata.Aggregation {
-	// TODO(#2971): implement.
-	return nil
+	gauge := metricdata.Gauge[N]{}
+
+	s.Lock()
+	defer s.Unlock()
+
+	if len(s.values) == 0 {
+		return gauge
+	}
+
+	gauge.DataPoints = make([]metricdata.DataPoint[N], 0, len(s.values))
+	for a, v := range s.values {
+		gauge.DataPoints = append(gauge.DataPoints, metricdata.DataPoint[N]{
+			Attributes: a,
+			// The event time is the only meaningful timestamp, StartTime is
+			// ignored.
+			Time:  v.timestamp,
+			Value: v.value,
+		})
+		// Do not report stale values.
+		delete(s.values, a)
+	}
+	return gauge
 }

sdk/metric/internal/lastvalue_test.go

@@ -0,0 +1,91 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build go1.18
+// +build go1.18
+
+package internal // import "go.opentelemetry.io/otel/sdk/metric/internal"
+
+import (
+	"testing"
+
+	"go.opentelemetry.io/otel/sdk/metric/metricdata"
+	"go.opentelemetry.io/otel/sdk/metric/metricdata/metricdatatest"
+)
+
+func TestLastValue(t *testing.T) {
+	t.Cleanup(mockTime(now))
+
+	t.Run("Int64", testLastValue[int64]())
+	t.Run("Float64", testLastValue[float64]())
+}
+
+func testLastValue[N int64 | float64]() func(*testing.T) {
+	tester := &aggregatorTester[N]{
+		GoroutineN:   defaultGoroutines,
+		MeasurementN: defaultMeasurements,
+		CycleN:       defaultCycles,
+	}
+
+	eFunc := func(increments setMap) expectFunc {
+		data := make([]metricdata.DataPoint[N], 0, len(increments))
+		for a, v := range increments {
+			point := metricdata.DataPoint[N]{Attributes: a, Time: now(), Value: N(v)}
+			data = append(data, point)
+		}
+		gauge := metricdata.Gauge[N]{DataPoints: data}
+		return func(int) metricdata.Aggregation { return gauge }
+	}
+	incr := monoIncr
+	return tester.Run(NewLastValue[N](), incr, eFunc(incr))
+}
+
+func testLastValueReset[N int64 | float64](t *testing.T) {
+	t.Cleanup(mockTime(now))
+
+	a := NewLastValue[N]()
+	expect := metricdata.Gauge[N]{}
+	metricdatatest.AssertAggregationsEqual(t, expect, a.Aggregation())
+
+	a.Aggregate(1, alice)
+	expect.DataPoints = []metricdata.DataPoint[N]{{
+		Attributes: alice,
+		Time:       now(),
+		Value:      1,
+	}}
+	metricdatatest.AssertAggregationsEqual(t, expect, a.Aggregation())
+
+	// The attr set should be forgotten once Aggregations is called.
+	expect.DataPoints = nil
+	metricdatatest.AssertAggregationsEqual(t, expect, a.Aggregation())
+
+	// Aggregating another set should not affect the original (alice).
+	a.Aggregate(1, bob)
+	expect.DataPoints = []metricdata.DataPoint[N]{{
+		Attributes: bob,
+		Time:       now(),
+		Value:      1,
+	}}
+	metricdatatest.AssertAggregationsEqual(t, expect, a.Aggregation())
+}
+
+func TestLastValueReset(t *testing.T) {
+	t.Run("Int64", testLastValueReset[int64])
+	t.Run("Float64", testLastValueReset[float64])
+}
+
+func BenchmarkLastValue(b *testing.B) {
+	b.Run("Int64", benchmarkAggregator(NewLastValue[int64]))
+	b.Run("Float64", benchmarkAggregator(NewLastValue[float64]))
+}