Pool attribute slices in stdouttrace self-observability (#7201)

### Benchmarks

```terminal
goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/exporters/stdout/stdouttrace
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
                      │  main.txt   │   stdouttrace-pool-obs-attrs.txt   │
                      │   sec/op    │   sec/op     vs base               │
ExporterExportSpans-8   30.55µ ± 2%   31.76µ ± 3%  +3.96% (p=0.000 n=10)

                      │   main.txt   │   stdouttrace-pool-obs-attrs.txt    │
                      │     B/op     │     B/op      vs base               │
ExporterExportSpans-8   6.538Ki ± 0%   6.353Ki ± 0%  -2.84% (p=0.000 n=10)

                      │  main.txt  │  stdouttrace-pool-obs-attrs.txt   │
                      │ allocs/op  │ allocs/op   vs base               │
ExporterExportSpans-8   135.0 ± 0%   134.0 ± 0%  -0.74% (p=0.000 n=10)
```

exporters/stdout/stdouttrace/trace.go

@@ -96,6 +96,17 @@ type Exporter struct {
 	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
+	},
+}
+
 // ExportSpans writes spans in json format to stdout.
 func (e *Exporter) ExportSpans(ctx context.Context, spans []trace.ReadOnlySpan) (err error) {
 	var success int64
@@ -116,11 +127,14 @@ func (e *Exporter) ExportSpans(ctx context.Context, spans []trace.ReadOnlySpan)
 			if err != nil {
 				// additional attributes for self-observability,
 				// only spanExportedMetric and operationDurationMetric are supported.
-				//
-				// TODO: use a pool to amortize allocations.
-				attr = make([]attribute.KeyValue, len(e.selfObservabilityAttrs), len(e.selfObservabilityAttrs)+1)
-				copy(attr, e.selfObservabilityAttrs)
-				attr = append(attr, semconv.ErrorType(err))
+				attrs := measureAttrsPool.Get().(*[]attribute.KeyValue)
+				defer func() {
+					*attrs = (*attrs)[:0] // reset the slice for reuse
+					measureAttrsPool.Put(attrs)
+				}()
+				*attrs = append(*attrs, e.selfObservabilityAttrs...)
+				*attrs = append(*attrs, semconv.ErrorType(err))
+				attr = *attrs
 
 				e.spanExportedMetric.Add(ctx, count-success, attr...)
 			}

exporters/stdout/stdouttrace/trace_test.go

@@ -667,3 +667,26 @@ func TestSelfObservabilityInstrumentErrors(t *testing.T) {
 	assert.ErrorContains(t, err, "span exported metric")
 	assert.ErrorContains(t, err, "operation duration metric")
 }
+
+func BenchmarkExporterExportSpans(b *testing.B) {
+	b.Setenv("OTEL_GO_X_SELF_OBSERVABILITY", "true")
+	ss := tracetest.SpanStubs{
+		{Name: "/foo"},
+		{
+			Name:       "JSON encoder cannot marshal math.Inf(1)",
+			Attributes: []attribute.KeyValue{attribute.Float64("", math.Inf(1))},
+		},
+		{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)
+	}
+	_ = err
+}