Split the span start/end benchmarks and test start with links and attributes (#5554)

I was looking at the trace benchmarks, and noticed this one, which says
it tests "span start", but ending the span is included within the data.
So this change removes ending the span from the computation, and adds a
new benchmark which only computes span end.

benchstat for span start:

```
pkg: go.opentelemetry.io/otel/sdk/trace
              │ bench-main  │            bench-branch            │
              │   sec/op    │   sec/op     vs base               │
TraceStart-10   725.6n ± 3%   667.2n ± 2%  -8.04% (p=0.000 n=10)

              │ bench-main │          bench-branch          │
              │    B/op    │    B/op     vs base            │
TraceStart-10   704.0 ± 0%   704.0 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

              │ bench-main │          bench-branch          │
              │ allocs/op  │ allocs/op   vs base            │
TraceStart-10   14.00 ± 0%   14.00 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal
```

Benchmark for span end:
```
BenchmarkSpanEnd-10     16486819               147.7 ns/op             0 B/op          0 allocs/op
```

sdk/trace/span_test.go

@@ -13,6 +13,7 @@ import (
 
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/codes"
+	"go.opentelemetry.io/otel/trace"
 )
 
 func TestSetStatus(t *testing.T) {
@@ -277,3 +278,20 @@ func BenchmarkRecordingSpanSetAttributes(b *testing.B) {
 		})
 	}
 }
+
+func BenchmarkSpanEnd(b *testing.B) {
+	tracer := NewTracerProvider().Tracer("")
+	ctx := trace.ContextWithSpanContext(context.Background(), trace.SpanContext{})
+
+	spans := make([]trace.Span, b.N)
+	for i := 0; i < b.N; i++ {
+		_, span := tracer.Start(ctx, "")
+		spans[i] = span
+	}
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		spans[i].End()
+	}
+}

sdk/trace/trace_test.go

@@ -2112,11 +2112,48 @@ func BenchmarkTraceStart(b *testing.B) {
 	tracer := NewTracerProvider().Tracer("")
 	ctx := trace.ContextWithSpanContext(context.Background(), trace.SpanContext{})
 
-	b.ReportAllocs()
-	b.ResetTimer()
+	l1 := trace.Link{SpanContext: trace.SpanContext{}, Attributes: []attribute.KeyValue{}}
+	l2 := trace.Link{SpanContext: trace.SpanContext{}, Attributes: []attribute.KeyValue{}}
 
-	for i := 0; i < b.N; i++ {
-		_, span := tracer.Start(ctx, "")
-		span.End()
+	links := []trace.Link{l1, l2}
+
+	for _, tt := range []struct {
+		name    string
+		options []trace.SpanStartOption
+	}{
+		{
+			name: "with a simple span",
+		},
+		{
+			name: "with several links",
+			options: []trace.SpanStartOption{
+				trace.WithLinks(links...),
+			},
+		},
+		{
+			name: "with attributes",
+			options: []trace.SpanStartOption{
+				trace.WithAttributes(
+					attribute.String("key1", "value1"),
+					attribute.String("key2", "value2"),
+				),
+			},
+		},
+	} {
+		b.Run(tt.name, func(b *testing.B) {
+			spans := make([]trace.Span, b.N)
+			b.ReportAllocs()
+			b.ResetTimer()
+
+			for i := 0; i < b.N; i++ {
+				_, span := tracer.Start(ctx, "", tt.options...)
+				spans[i] = span
+			}
+
+			b.StopTimer()
+			for i := 0; i < b.N; i++ {
+				spans[i].End()
+			}
+		})
 	}
 }