fixup: add hint when running with codspeed instrumentation

Author: not-matthias

testing/testing/benchmark.go

@@ -280,6 +280,7 @@ var labelsOnce sync.Once
 // subbenchmarks. b must not have subbenchmarks.
 func (b *B) run() {
 	labelsOnce.Do(func() {
+		fmt.Fprintf(b.w, "Running with codspeed instrumentation\n")
 		fmt.Fprintf(b.w, "goos: %s\n", runtime.GOOS)
 		fmt.Fprintf(b.w, "goarch: %s\n", runtime.GOARCH)
 		if b.importPath != "" {