runtime/trace: add Log benchmark

This is a pretty decent benchmark of the baseline event cost.

Change-Id: I22a7fa3411bd80be3bd8093d5933e29062cb1377
Reviewed-on: https://go-review.googlesource.com/c/go/+/723060
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/trace/annotation_test.go

@@ -6,6 +6,7 @@ package trace_test
 
 import (
 	"context"
+	"io"
 	. "runtime/trace"
 	"testing"
 )
@@ -34,3 +35,15 @@ func BenchmarkNewTask(b *testing.B) {
 		}
 	})
 }
+
+func BenchmarkLog(b *testing.B) {
+	b.ReportAllocs()
+
+	Start(io.Discard)
+	defer Stop()
+
+	ctx := context.Background()
+	for b.Loop() {
+		Log(ctx, "", "")
+	}
+}