Allow an operation on an endpoint to specify the media type that it produces

[wilkinsona]

The Prometheus endpoint that's proposed in #9970 needs to provide an operation that returns text/plain; version=0.0.4; charset=utf-8. There's no way to do so with the current web endpoint infrastructure.

[wilkinsona]

This is, arguably, also applicable to JMX. At the moment, any response from an endpoint that isn't already a String is serialised into JSON and then returned as a String. Rather than always using JSON, the media type produced by the operation could be used to influence that conversion.

This possible enhancement shouldn't be tackled in this issue, but the design should consider the fact that knowing what media type an operation produces isn't, necessarily, specific to web endpoints.