This is a follow-up to comments on #338 which has added echo_request_headers to the test server filter ResponseOptions.

I'd like to add more response options that I think might be useful:

• echo_request_metadata - include the dump of request metadata in the response body
• echo_request_body - include the body of POST request in the response body
• status_code - set http status code to arbitrary value, like 204 or 302

@htuch has suggested that we might want to eventually have a more structured body. E.g. make it a proto defined JSON format or something like that.

I think that makes sense, and I'd like to have more input.

• Should echo response be included into response body or special response headers? I think that including response in headers are nice because they could be stripped automatically AND are easily parsed. E.g. x-echo-request-headers: { some_headers_dump }.

• What format do we want structured response to use? Arbitrary JSON? Some specifically defined proto? Something else?

Any other ideas / suggestions?