sendMetadata (md) {
    // if forcing send reset our metadata
    if (md && (typeof md === 'object' || md instanceof grpc.Metadata)) {
      this.metadata = null
      this.set(md)
    }

    const data = this.getMetadata()
    if (data) {
      this.call.sendMetadata(data)
    }
  }
Sends the response header metadata. Optionally (re)sets the header metadata as well.
@param {Object} md optional header metadata object to set into the request before sending
                   if there is existing metadata in the response it is cleared
                   if param is not provided `sendMetadata` sends the existing metadata in the response
@example <caption>Set and send</caption>
ctx.response.sendMetadata({
  foo: 'bar'
})
@example <caption>Set and send later</caption>
ctx.response.set('foo', 'bar')
// ... later
ctx.response.sendMetadata()