feat(server): echo request headers in response over REST transport

[vchudnov-g]

Towards #1493, but note that this will occur for all Showcase services, not just Echo.

This can be verified manually on the shell via:

❯ curl -i -X POST -H "X-Goog-Api-Client: rest/0.0.0 gapic/0.0.0" -H 'Content-Type: application/json' http://localhost:7469/v1beta1/echo:echo -d'{"content":"g
reetings"}'                                                                                                                                                 
HTTP/1.1 200 OK
X-Showcase-Request-Accept: */*
X-Showcase-Request-Content-Length: 23
X-Showcase-Request-Content-Type: application/json
X-Showcase-Request-User-Agent: curl/8.5.0
X-Showcase-Request-X-Goog-Api-Client: rest/0.0.0 gapic/0.0.0
Date: Fri, 26 Apr 2024 21:33:24 GMT
Content-Length: 104
Content-Type: text/plain; charset=utf-8

{
  "content":  "greetings",
  "severity":  "UNNECESSARY",
  "requestId":  "",
  "otherRequestId":  ""
}

and works even for error responses:

❯ curl -i -X POST -H "X-Goog-Api-Client: rest/0.0.0 gapic/0.0.0" -H 'Content-Type: application/json' http://localhost:7469/v1beta1/echo:echo -d'{"foo":"greet
ings"}'                                                                                                                                                     
HTTP/1.1 400 Bad Request
X-Showcase-Request-Accept: */*
X-Showcase-Request-Content-Length: 19
X-Showcase-Request-Content-Type: application/json
X-Showcase-Request-User-Agent: curl/8.5.0
X-Showcase-Request-X-Goog-Api-Client: rest/0.0.0 gapic/0.0.0
Date: Fri, 26 Apr 2024 21:47:05 GMT
Content-Length: 160
Content-Type: text/plain; charset=utf-8

{"error":{"code":400,"message":"error reading body params '*': proto: (line 1:2): unknown field \"foo\"","details":null,"Body":"","Header":null,"Errors":null}

[parthea]

LGTM. I tested this locally using the code in googleapis/gapic-generator-python#1999

[noahdietz]

FWIW I don't think @vchudnov-g had landed on a header prefix yet, at least the suggestions in the document he wrote are different than what is implemented here...

[parthea]

At the time of merging this PR it seemed like we had settled on a prefix based on the comment thread in the document.

[noahdietz]

At the time of merging this PR it seemed like we had settled on a prefix based on the comment thread in the document.

You're right, @vchudnov-g had settled on that!

[vchudnov-g]

One thought that occurs to me: maybe for the same reasons we have prefixes in the echoed http headers, we should also have them in the gRPC trailers that echo the gRPC request headers? Thoughts?

[noahdietz]

One thought that occurs to me: maybe for the same reasons we have prefixes in the echoed http headers, we should also have them in the gRPC trailers that echo the gRPC request headers? Thoughts?

Yeah it probably makes sense to do, but let's make sure we blast it out in release notes for those that already have trailer tests.