Add the SampledFilter exemplar Reservoir impl (#4851)

This Reservoir implementaiton is used at the MeterProvider level to
pre-filter measurements offered to a wrapped Reservoir.

Co-authored-by: Robert Pająk <pellared@hotmail.com>

sdk/metric/internal/exemplar/filter.go

@@ -0,0 +1,40 @@
+// 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.
+
+package exemplar // import "go.opentelemetry.io/otel/sdk/metric/internal/exemplar"
+
+import (
+	"context"
+	"time"
+
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/trace"
+)
+
+// SampledFilter returns a [Reservoir] wrapping r that will only offer measurements
+// to r if the passed context associated with the measurement contains a sampled
+// [go.opentelemetry.io/otel/trace.SpanContext].
+func SampledFilter[N int64 | float64](r Reservoir[N]) Reservoir[N] {
+	return filtered[N]{Reservoir: r}
+}
+
+type filtered[N int64 | float64] struct {
+	Reservoir[N]
+}
+
+func (f filtered[N]) Offer(ctx context.Context, t time.Time, n N, a []attribute.KeyValue) {
+	if trace.SpanContextFromContext(ctx).IsSampled() {
+		f.Reservoir.Offer(ctx, t, n, a)
+	}
+}

sdk/metric/internal/exemplar/filter_test.go

@@ -0,0 +1,77 @@
+// 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.
+
+package exemplar // import "go.opentelemetry.io/otel/sdk/metric/internal/exemplar"
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/sdk/metric/metricdata"
+	"go.opentelemetry.io/otel/trace"
+)
+
+func TestSampledFilter(t *testing.T) {
+	t.Run("Int64", testSampledFiltered[int64])
+	t.Run("Float64", testSampledFiltered[float64])
+}
+
+func testSampledFiltered[N int64 | float64](t *testing.T) {
+	under := &res[N]{}
+
+	r := SampledFilter[N](under)
+
+	ctx := context.Background()
+	r.Offer(ctx, staticTime, 0, nil)
+	assert.False(t, under.OfferCalled, "underlying Reservoir Offer called")
+	r.Offer(sample(ctx), staticTime, 0, nil)
+	assert.True(t, under.OfferCalled, "underlying Reservoir Offer not called")
+
+	r.Collect(nil)
+	assert.True(t, under.CollectCalled, "underlying Reservoir Collect not called")
+
+	r.Flush(nil)
+	assert.True(t, under.FlushCalled, "underlying Reservoir Flush not called")
+}
+
+func sample(parent context.Context) context.Context {
+	sc := trace.NewSpanContext(trace.SpanContextConfig{
+		TraceID:    trace.TraceID{0x01},
+		SpanID:     trace.SpanID{0x01},
+		TraceFlags: trace.FlagsSampled,
+	})
+	return trace.ContextWithSpanContext(parent, sc)
+}
+
+type res[N int64 | float64] struct {
+	OfferCalled   bool
+	CollectCalled bool
+	FlushCalled   bool
+}
+
+func (r *res[N]) Offer(context.Context, time.Time, N, []attribute.KeyValue) {
+	r.OfferCalled = true
+}
+
+func (r *res[N]) Collect(*[]metricdata.Exemplar[N]) {
+	r.CollectCalled = true
+}
+
+func (r *res[N]) Flush(*[]metricdata.Exemplar[N]) {
+	r.FlushCalled = true
+}