Custom metadata to be sent to the client. Will be null once the headers
have been sent, either when sendHeaders is called, or when the first
response message is sent.
Send response headers. This is done automatically before sending the first
response message, but can be done manually before the first response is
ready, if necessary.
Send response trailers. A trailer indicating success (status == 0) will
be sent automatically when all responses are sent. This method can be used
to send a different status code, if needed.