Add method and benchmarks for pooling metric options (#6394)

This PR improves the `RecordMetrics()` allocations using a pool for the
`metric.AddOption` slice.

`benchstat main.txt pool.txt`:
```
goos: darwin
goarch: arm64
pkg: go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp/internal/semconv
cpu: Apple M1 Max
                            │  main.txt   │              pool.txt              │
                            │   sec/op    │   sec/op     vs base               │
RecordMetrics/empty-10        4.513n ± 3%   4.620n ± 0%  +2.38% (p=0.037 n=10)
RecordMetrics/nil_meter-10    341.8n ± 1%   330.7n ± 4%  -3.26% (p=0.022 n=10)
RecordMetrics/with_Meter-10   341.1n ± 1%   330.0n ± 1%  -3.27% (p=0.000 n=10)
geomean                       80.74n        79.59n       -1.42%

                            │   main.txt   │              pool.txt               │
                            │     B/op     │    B/op     vs base                 │
RecordMetrics/empty-10        0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
RecordMetrics/nil_meter-10    696.0 ± 0%     680.0 ± 0%  -2.30% (p=0.000 n=10)
RecordMetrics/with_Meter-10   696.0 ± 0%     680.0 ± 0%  -2.30% (p=0.000 n=10)
geomean                                  ²               -1.54%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean

                            │   main.txt   │               pool.txt               │
                            │  allocs/op   │ allocs/op   vs base                  │
RecordMetrics/empty-10        0.000 ± 0%     0.000 ± 0%        ~ (p=1.000 n=10) ¹
RecordMetrics/nil_meter-10    6.000 ± 0%     5.000 ± 0%  -16.67% (p=0.000 n=10)
RecordMetrics/with_Meter-10   6.000 ± 0%     5.000 ± 0%  -16.67% (p=0.000 n=10)
geomean                                  ²               -11.45%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean
```

Fixes
#5968

---------

Co-authored-by: Damien Mathieu <[email protected]>
Co-authored-by: Tyler Yahn <[email protected]>

CHANGELOG.md

@@ -15,6 +15,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - The `go.opentelemetry.io/contrib/bridges/otellogr` module.
   This module provides an OpenTelemetry logging bridge for `github.com/go-logr/logr`. (#6386)
 - Added SNS instrumentation in `go.opentelemetry.io/contrib/instrumentation/github.com/aws/aws-sdk-go-v2/otelaws`. (#6388)
+- Use a `sync.Pool` for metric options in `go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp`. (#6394)
 
 ### Changed
 

instrumentation/net/http/otelhttp/internal/semconv/env.go

@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"os"
 	"strings"
+	"sync"
 
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/codes"
@@ -102,6 +103,12 @@ type MetricData struct {
 	ElapsedTime float64
 }
 
+var metricAddOptionPool = &sync.Pool{
+	New: func() interface{} {
+		return &[]metric.AddOption{}
+	},
+}
+
 func (s HTTPServer) RecordMetrics(ctx context.Context, md ServerMetricData) {
 	if s.requestBytesCounter == nil || s.responseBytesCounter == nil || s.serverLatencyMeasure == nil {
 		// This will happen if an HTTPServer{} is used instead of NewHTTPServer.
@@ -110,10 +117,13 @@ func (s HTTPServer) RecordMetrics(ctx context.Context, md ServerMetricData) {
 
 	attributes := OldHTTPServer{}.MetricAttributes(md.ServerName, md.Req, md.StatusCode, md.AdditionalAttributes)
 	o := metric.WithAttributeSet(attribute.NewSet(attributes...))
-	addOpts := []metric.AddOption{o}
-	s.requestBytesCounter.Add(ctx, md.RequestSize, addOpts...)
-	s.responseBytesCounter.Add(ctx, md.ResponseSize, addOpts...)
+	addOpts := metricAddOptionPool.Get().(*[]metric.AddOption)
+	*addOpts = append(*addOpts, o)
+	s.requestBytesCounter.Add(ctx, md.RequestSize, *addOpts...)
+	s.responseBytesCounter.Add(ctx, md.ResponseSize, *addOpts...)
 	s.serverLatencyMeasure.Record(ctx, md.ElapsedTime, o)
+	*addOpts = (*addOpts)[:0]
+	metricAddOptionPool.Put(addOpts)
 
 	// TODO: Duplicate Metrics
 }

instrumentation/net/http/otelhttp/internal/semconv/env_test.go

@@ -90,3 +90,42 @@ func TestHTTPClientDoesNotPanic(t *testing.T) {
 		})
 	}
 }
+
+func BenchmarkRecordMetrics(b *testing.B) {
+	benchmarks := []struct {
+		name   string
+		server HTTPServer
+	}{
+		{
+			name:   "empty",
+			server: HTTPServer{},
+		},
+		{
+			name:   "nil meter",
+			server: NewHTTPServer(nil),
+		},
+		{
+			name:   "with Meter",
+			server: NewHTTPServer(noop.Meter{}),
+		},
+	}
+
+	for _, bm := range benchmarks {
+		b.Run(bm.name, func(b *testing.B) {
+			req, _ := http.NewRequest("GET", "http://example.com", nil)
+			_ = bm.server.RequestTraceAttrs("stuff", req)
+			_ = bm.server.ResponseTraceAttrs(ResponseTelemetry{StatusCode: 200})
+			ctx := context.Background()
+			b.ReportAllocs()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				bm.server.RecordMetrics(ctx, ServerMetricData{
+					ServerName: bm.name,
+					MetricAttributes: MetricAttributes{
+						Req: req,
+					},
+				})
+			}
+		})
+	}
+}