code: func tagKey(kv string) string {
  	if idx := strings.IndexRune(kv, ':'); idx != -1 {
  		return kv[:idx]
  	}
  	return kv
  }
docs: // tagKey takes "key:<stuff>" and returns "key".
  //
  // It is a faster version of common.ParseInstanceTags that skips validation
  // (since stored entities are already validated).
  //
  // We micro-optimized this part because comparator in ListInstanceTags is a hot
  // spot when sorting a lot of entities.