Amortize measurement option allocations (#7215)

This is not a performance critical exporter, but it is acting as the
foundation for all other self-observability work. We want to ensure
performance is considered for all this other work.

- Do not allocate the add and record measurement option slices.
- Do not allocate the `metric.attrOpt` to head when on default path
(i.e. `WithAttributeSet` or `WithAttributes`)

There are three additional allocations in the self-observability. It
appears these are added in the error scenario where we need to
dynamically build the attributes that are being recorded.

### Benchmarks

```terminal
$ benchstat main-49be00144.txt amortize-opts-stdouttrace.txt
goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/exporters/stdout/stdouttrace
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
                                        │ main-49be00144.txt │   amortize-opts-stdouttrace.txt    │
                                        │       sec/op       │   sec/op     vs base               │
ExporterExportSpans/SelfObservability-8          26.89µ ± 3%   27.40µ ± 3%       ~ (p=0.143 n=10)
ExporterExportSpans/NoObservability-8            25.99µ ± 3%   27.22µ ± 2%  +4.76% (p=0.004 n=10)

                                        │ main-49be00144.txt │    amortize-opts-stdouttrace.txt     │
                                        │        B/op        │     B/op      vs base                │
ExporterExportSpans/SelfObservability-8         5.459Ki ± 0%   4.081Ki ± 0%  -25.24% (p=0.000 n=10)
ExporterExportSpans/NoObservability-8           3.873Ki ± 0%   3.873Ki ± 0%        ~ (p=1.000 n=10)

                                        │ main-49be00144.txt │    amortize-opts-stdouttrace.txt     │
                                        │     allocs/op      │ allocs/op   vs base                  │
ExporterExportSpans/SelfObservability-8           80.00 ± 0%   67.00 ± 0%  -16.25% (p=0.000 n=10)
ExporterExportSpans/NoObservability-8             65.00 ± 0%   65.00 ± 0%        ~ (p=1.000 n=10) ¹
¹ all samples are equal
```

## Follow-up

- Investigate if the `semconv` helper packages can be updated to accept
some of this complexity (e.g. add a `AddSet` method that accepts an
`attribute.Set` instead of just `...attribute.KeyValue`).

## Alternatives

A cleaner version is found in
#7226. That
relies on an external [`bind`](https://github.com/MrAlias/bind) package
and the clean up mentioned above.

Author: MrAlias

.codespellignore

@@ -7,3 +7,4 @@ ans
 nam
 valu
 thirdparty
+addOpt

exporters/stdout/stdouttrace/trace.go

@@ -55,6 +55,8 @@ func New(options ...Option) (*Exporter, error) {
 		semconv.OTelComponentName(fmt.Sprintf("%s/%d", otelComponentType, counter.NextExporterID())),
 		semconv.OTelComponentTypeKey.String(otelComponentType),
 	}
+	s := attribute.NewSet(exporter.selfObservabilityAttrs...)
+	exporter.selfObservabilitySetOpt = metric.WithAttributeSet(s)
 
 	mp := otel.GetMeterProvider()
 	m := mp.Meter(
@@ -91,39 +93,66 @@ type Exporter struct {
 
 	selfObservabilityEnabled bool
 	selfObservabilityAttrs   []attribute.KeyValue // selfObservability common attributes
+	selfObservabilitySetOpt  metric.MeasurementOption
 	spanInflightMetric       otelconv.SDKExporterSpanInflight
 	spanExportedMetric       otelconv.SDKExporterSpanExported
 	operationDurationMetric  otelconv.SDKExporterOperationDuration
 }
 
-var measureAttrsPool = sync.Pool{
-	New: func() any {
-		// "component.name" + "component.type" + "error.type"
-		const n = 1 + 1 + 1
-		s := make([]attribute.KeyValue, 0, n)
-		// Return a pointer to a slice instead of a slice itself
-		// to avoid allocations on every call.
-		return &s
-	},
-}
+var (
+	measureAttrsPool = sync.Pool{
+		New: func() any {
+			// "component.name" + "component.type" + "error.type"
+			const n = 1 + 1 + 1
+			s := make([]attribute.KeyValue, 0, n)
+			// Return a pointer to a slice instead of a slice itself
+			// to avoid allocations on every call.
+			return &s
+		},
+	}
+
+	addOptPool = &sync.Pool{
+		New: func() any {
+			const n = 1 // WithAttributeSet
+			o := make([]metric.AddOption, 0, n)
+			return &o
+		},
+	}
+
+	recordOptPool = &sync.Pool{
+		New: func() any {
+			const n = 1 // WithAttributeSet
+			o := make([]metric.RecordOption, 0, n)
+			return &o
+		},
+	}
+)
 
 // ExportSpans writes spans in json format to stdout.
 func (e *Exporter) ExportSpans(ctx context.Context, spans []trace.ReadOnlySpan) (err error) {
 	var success int64
 	if e.selfObservabilityEnabled {
 		count := int64(len(spans))
 
-		e.spanInflightMetric.Add(ctx, count, e.selfObservabilityAttrs...)
+		addOpt := addOptPool.Get().(*[]metric.AddOption)
+		defer func() {
+			*addOpt = (*addOpt)[:0]
+			addOptPool.Put(addOpt)
+		}()
+
+		*addOpt = append(*addOpt, e.selfObservabilitySetOpt)
+
+		e.spanInflightMetric.Inst().Add(ctx, count, *addOpt...)
 		defer func(starting time.Time) {
-			e.spanInflightMetric.Add(ctx, -count, e.selfObservabilityAttrs...)
+			e.spanInflightMetric.Inst().Add(ctx, -count, *addOpt...)
 
 			// Record the success and duration of the operation.
 			//
 			// Do not exclude 0 values, as they are valid and indicate no spans
 			// were exported which is meaningful for certain aggregations.
-			e.spanExportedMetric.Add(ctx, success, e.selfObservabilityAttrs...)
+			e.spanExportedMetric.Inst().Add(ctx, success, *addOpt...)
 
-			attr := e.selfObservabilityAttrs
+			mOpt := e.selfObservabilitySetOpt
 			if err != nil {
 				// additional attributes for self-observability,
 				// only spanExportedMetric and operationDurationMetric are supported.
@@ -134,12 +163,34 @@ func (e *Exporter) ExportSpans(ctx context.Context, spans []trace.ReadOnlySpan)
 				}()
 				*attrs = append(*attrs, e.selfObservabilityAttrs...)
 				*attrs = append(*attrs, semconv.ErrorType(err))
-				attr = *attrs
 
-				e.spanExportedMetric.Add(ctx, count-success, attr...)
+				// Do not inefficiently make a copy of attrs by using
+				// WithAttributes instead of WithAttributeSet.
+				set := attribute.NewSet(*attrs...)
+				mOpt = metric.WithAttributeSet(set)
+
+				// Reset addOpt with new attribute set.
+				*addOpt = append((*addOpt)[:0], mOpt)
+
+				e.spanExportedMetric.Inst().Add(
+					ctx,
+					count-success,
+					*addOpt...,
+				)
 			}
 
-			e.operationDurationMetric.Record(ctx, time.Since(starting).Seconds(), attr...)
+			recordOpt := recordOptPool.Get().(*[]metric.RecordOption)
+			defer func() {
+				*recordOpt = (*recordOpt)[:0]
+				recordOptPool.Put(recordOpt)
+			}()
+
+			*recordOpt = append(*recordOpt, mOpt)
+			e.operationDurationMetric.Inst().Record(
+				ctx,
+				time.Since(starting).Seconds(),
+				*recordOpt...,
+			)
 		}(time.Now())
 	}
 

exporters/stdout/stdouttrace/trace_test.go

@@ -669,7 +669,6 @@ func TestSelfObservabilityInstrumentErrors(t *testing.T) {
 }
 
 func BenchmarkExporterExportSpans(b *testing.B) {
-	b.Setenv("OTEL_GO_X_SELF_OBSERVABILITY", "true")
 	ss := tracetest.SpanStubs{
 		{Name: "/foo"},
 		{
@@ -678,15 +677,25 @@ func BenchmarkExporterExportSpans(b *testing.B) {
 		},
 		{Name: "/bar"},
 	}.Snapshots()
-	ex, err := stdouttrace.New(stdouttrace.WithWriter(io.Discard))
-	if err != nil {
-		b.Fatalf("failed to create exporter: %v", err)
-	}
 
-	b.ReportAllocs()
-	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
-		err = ex.ExportSpans(context.Background(), ss)
+	run := func(b *testing.B) {
+		ex, err := stdouttrace.New(stdouttrace.WithWriter(io.Discard))
+		if err != nil {
+			b.Fatalf("failed to create exporter: %v", err)
+		}
+
+		b.ReportAllocs()
+		b.ResetTimer()
+		for i := 0; i < b.N; i++ {
+			err = ex.ExportSpans(context.Background(), ss)
+		}
+		_ = err
 	}
-	_ = err
+
+	b.Run("SelfObservability", func(b *testing.B) {
+		b.Setenv("OTEL_GO_X_SELF_OBSERVABILITY", "true")
+		run(b)
+	})
+
+	b.Run("NoObservability", run)
 }