trace: Span in noop.Start is no longer allocated (#5457)

trace/noop/noop.Span should be instance. This will reduce memory waste.

Benchmark results

with changes:
new.txt
```
goos: darwin
goarch: arm64
pkg: go.opentelemetry.io/otel/trace/noop
BenchmarkNoopInstance-10        29894822                39.94 ns/op           48 B/op          1 allocs/op
BenchmarkNoopInstance-10        29675424                39.99 ns/op           48 B/op          1 allocs/op
BenchmarkNoopInstance-10        30064700                39.98 ns/op           48 B/op          1 allocs/op
BenchmarkNoopInstance-10        29962016                40.03 ns/op           48 B/op          1 allocs/op
BenchmarkNoopInstance-10        30060465                40.02 ns/op           48 B/op          1 allocs/op
BenchmarkNoopInstance-10        29916855                40.04 ns/op           48 B/op          1 allocs/op
BenchmarkNoopInstance-10        29829998                40.28 ns/op           48 B/op          1 allocs/op
BenchmarkNoopInstance-10        30084706                39.99 ns/op           48 B/op          1 allocs/op
BenchmarkNoopInstance-10        30087441                40.02 ns/op           48 B/op          1 allocs/op
BenchmarkNoopInstance-10        29864365                40.14 ns/op           48 B/op          1 allocs/op
```
without changes on `main`:
old.txt
```
goos: darwin
goarch: arm64
pkg: go.opentelemetry.io/otel/trace/noop
BenchmarkNoopInstance-10        14813442                67.64 ns/op          128 B/op          2 allocs/op
BenchmarkNoopInstance-10        17714486                68.17 ns/op          128 B/op          2 allocs/op
BenchmarkNoopInstance-10        17701257                67.66 ns/op          128 B/op          2 allocs/op
BenchmarkNoopInstance-10        17805859                67.69 ns/op          128 B/op          2 allocs/op
BenchmarkNoopInstance-10        17912841                67.43 ns/op          128 B/op          2 allocs/op
BenchmarkNoopInstance-10        17864120                67.58 ns/op          128 B/op          2 allocs/op
BenchmarkNoopInstance-10        17663130                68.41 ns/op          128 B/op          2 allocs/op
BenchmarkNoopInstance-10        17740423                67.57 ns/op          128 B/op          2 allocs/op
BenchmarkNoopInstance-10        17751040                67.56 ns/op          128 B/op          2 allocs/op
BenchmarkNoopInstance-10        17738064                67.91 ns/op          128 B/op          2 allocs/op

```

benchstat: 
```
goos: darwin
goarch: arm64
pkg: go.opentelemetry.io/otel/trace/noop
                │   old.txt   │               new.txt               │
                │   sec/op    │   sec/op     vs base                │
NoopInstance-10   67.65n ± 1%   40.02n ± 0%  -40.84% (p=0.000 n=10)

                │   old.txt   │              new.txt               │
                │    B/op     │    B/op     vs base                │
NoopInstance-10   128.00 ± 0%   48.00 ± 0%  -62.50% (p=0.000 n=10)

                │  old.txt   │              new.txt               │
                │ allocs/op  │ allocs/op   vs base                │
NoopInstance-10   2.000 ± 0%   1.000 ± 0%  -50.00% (p=0.000 n=10)
```

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

CHANGELOG.md

@@ -13,6 +13,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - The `IsEmpty` method is added to the `Instrument` type in `go.opentelemetry.io/otel/sdk/metric`.
   This method is used to check if an `Instrument` instance is a zero-value. (#5431)
 
+### Changed
+
+- `Tracer.Start` in `go.opentelemetry.io/otel/trace/noop` no longer allocates a span for empty span context. (#5457)
+
 ### Fixed
 
 - Log a warning to the OpenTelemetry internal logger when a `Record` in `go.opentelemetry.io/otel/sdk/log` drops an attribute due to a limit being reached. (#5376)

trace/noop/noop.go

@@ -67,11 +67,13 @@ func (t Tracer) Start(ctx context.Context, _ string, _ ...trace.SpanStartOption)
 		span = Span{sc: sc}
 	} else {
 		// No parent, return a No-Op span with an empty span context.
-		span = Span{}
+		span = noopSpanInstance
 	}
 	return trace.ContextWithSpan(ctx, span), span
 }
 
+var noopSpanInstance trace.Span = Span{}
+
 // Span is an OpenTelemetry No-Op Span.
 type Span struct {
 	embedded.Span

trace/noop/noop_test.go

@@ -101,6 +101,19 @@ func TestTracerStartPropagatesSpanContext(t *testing.T) {
 	assert.False(t, span.IsRecording(), "recording span returned")
 }
 
+func BenchmarkNoopInstance(b *testing.B) {
+	tracer := NewTracerProvider().Tracer("")
+	ctx := trace.ContextWithSpanContext(context.Background(), trace.SpanContext{})
+
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		_, span := tracer.Start(ctx, "")
+		span.End()
+	}
+}
+
 type recordingSpan struct{ Span }
 
 func (recordingSpan) IsRecording() bool { return true }