code: func (m metadataTextMap) Set(key, val string) {
  	// gRPC allows for complex binary values to be written.
  	encodedKey, encodedVal := encodeKeyValue(key, val)
  	// The metadata object is a multimap, and previous values may exist, but for opentracing headers, we do not append
  	// we just override.
  	m[encodedKey] = []string{encodedVal}
  }
docs: // Set is a opentracing.TextMapReader interface that extracts values.