code: func LogMessage(message string, keyvals ...keyval) *logmessage {
  	return &logmessage{m: message, kv: keyvals}
  }
docs: // LogMessage creates a message that complies with fmt.Stringer but also
  // includes a message and key/value pairs for structured logging.
  // Use it in trace.LazyLog:
  //
  // 		t.LazyLog(trace.LogMessage("found", trace.KeyValue("file", name)), false)