Add prometheus exporter benchmark (#3213)

* add prometheus exporter benchmark

* Update exporters/prometheus/benchmark_test.go

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

* Update exporters/prometheus/benchmark_test.go

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>
Co-authored-by: Chester Cheung <cheung.zhy.csu@gmail.com>
Co-authored-by: Tyler Yahn <codingalias@gmail.com>

exporters/prometheus/benchmark_test.go

@@ -0,0 +1,56 @@
+// 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 prometheus
+
+import (
+	"context"
+	"fmt"
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/stretchr/testify/require"
+
+	"go.opentelemetry.io/otel/sdk/metric"
+)
+
+func benchmarkCollect(b *testing.B, n int) {
+	ctx := context.Background()
+	exporter := New()
+	provider := metric.NewMeterProvider(metric.WithReader(exporter))
+	meter := provider.Meter("testmeter")
+
+	registry := prometheus.NewRegistry()
+	err := registry.Register(exporter.Collector)
+	require.NoError(b, err)
+
+	for i := 0; i < n; i++ {
+		counter, err := meter.SyncFloat64().Counter(fmt.Sprintf("foo_%d", i))
+		require.NoError(b, err)
+		counter.Add(ctx, float64(i))
+	}
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, err := registry.Gather()
+		require.NoError(b, err)
+	}
+}
+
+func BenchmarkCollect1(b *testing.B)     { benchmarkCollect(b, 1) }
+func BenchmarkCollect10(b *testing.B)    { benchmarkCollect(b, 10) }
+func BenchmarkCollect100(b *testing.B)   { benchmarkCollect(b, 100) }
+func BenchmarkCollect1000(b *testing.B)  { benchmarkCollect(b, 1000) }
+func BenchmarkCollect10000(b *testing.B) { benchmarkCollect(b, 10000) }