code: static String toAnnotation(Map<String, ?> fields) {
      // special-case the "event" field which is similar to the semantics of a zipkin annotation
      Object event = fields.get("event");
      if (event != null && fields.size() == 1) return event.toString();
  
      return joinOnEqualsSpace(fields);
    }
docs: Converts a map to a string of form: "key1=value1 key2=value2"