code: func GetServiceVersion(c context.Context, defaultVer string) string {
  	if md, ok := metadata.FromIncomingContext(c); ok {
  		values := md[VersionMetadataKey]
  		if len(values) != 0 {
  			return values[0]
  		}
  	}
  	return defaultVer
  }
docs: // GetServiceVersion extracts requested service version from metadata in c.