async def send_request(self):
        """Coroutine to send request headers with metadata to the server.

        New HTTP/2 stream will be created during this coroutine call.

        .. note:: This coroutine will be called implicitly during first
            :py:meth:`send_message` coroutine call, if not called before
            explicitly.
        """
        if self._send_request_done:
            raise ProtocolError('Request is already sent')

        with self._wrapper:
            protocol = await self._channel.__connect__()
            stream = protocol.processor.connection\
                .create_stream(wrapper=self._wrapper)

            headers = [
                (':method', 'POST'),
                (':scheme', self._channel._scheme),
                (':path', self._method_name),
                (':authority', self._channel._authority),
            ]
            if self._deadline is not None:
                timeout = self._deadline.time_remaining()
                headers.append(('grpc-timeout', encode_timeout(timeout)))
            content_type = (GRPC_CONTENT_TYPE
                            + '+' + self._codec.__content_subtype__)
            headers.extend((
                ('te', 'trailers'),
                ('content-type', content_type),
                ('user-agent', USER_AGENT),
            ))
            metadata, = await self._dispatch.send_request(
                self._metadata,
                method_name=self._method_name,
                deadline=self._deadline,
                content_type=content_type,
            )
            headers.extend(encode_metadata(metadata))
            release_stream = await stream.send_request(
                headers, _processor=protocol.processor,
            )
            self._stream = stream
            self._release_stream = release_stream
            self._send_request_done = True
Coroutine to send request headers with metadata to the server.

        New HTTP/2 stream will be created during this coroutine call.

        .. note:: This coroutine will be called implicitly during first
            :py:meth:`send_message` coroutine call, if not called before
            explicitly.