Before this change, on a trailers-only response, Metadata Exchange needed by CSM would just be dropped, and hence CSM labels would not be seen.
Changes -
* OTel call attempt tracer populates labels from trailers if it's a trailers-only response.
* HTTP2 layer propagates the Metadata Exchange field from headers to trailers for trailers-only responses.
* Add a test to make sure that retries continue to work when Metadata Exchange is enabled and a trailers-only response is sent. (This verifies that a trailers-only response remains a trailers-only response.)
Closes#36413
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36413 from yashykt:MetadataExchangeInTrailers e7d202685e
PiperOrigin-RevId: 630144618
108ee944df