Add Trailers.apply to construct from code, message, and details

[alexklibisz]

The handler generated by akka-grpc takes an error handler in the form of eHandler: ActorSystem => PartialFunction[Throwable, Trailers].

This change just adds a method to make it easier to construct an instance of Trailers containing rich error details, so that we can enrich the error response from within the error handler.

Couple things I'm not sure about in the initial implementation:

1. This implementation delegates to the analogous GrpcServiceException.apply method. Is there a preferred way to would abstract out the common code? I.e., create a Utils object somewhere.
2. The runtime/test submodule does not seem to import any instances of GeneratedMessage, so I'm not sure how to test that we're encoding details correctly. Should we modify the subproject so that it imports LocalizedMessage? Should we put the tests in another submodule.

[johanandren]

A lot of the test coverage is in the akka-grpc-plugin-tester-{java,scala} modules which have more access to actual message generation etc. so maybe sneak in some coverage there in LoggingErrorHandlingGreeterServer.java and LoggingErrorHandlingGreeterServer.scala. Those also has snippets that goes in the docs which may be useful to show off the new signature. For example the custom-error-mapping snippet.

Looks like there is no actual test calling those, but since this mostly adds convenience and no real logic having it used in something that is compiled as a part of the test suite is probably enough.

[alexklibisz]

I added the create method in ed7a913

And added an example to the LoggingErrorHandlingGreeterServer in 1885652.

If that example looks good, I can do the same on the Java side.

[johanandren]

That example looks good, please add for Java as well 👍

[alexklibisz]

Ran into an issue in the Java example. Commented inline.

[alexklibisz]

I'm seeing an error in the Gradle build: https://github.com/akka/akka-grpc/actions/runs/8851059859/job/24306757026?pr=1933#step:9:131

Error:  /home/runner/work/akka-grpc/akka-grpc/plugin-tester-scala/src/main/scala/example/myapp/helloworld/LoggingErrorHandlingGreeterServer.scala:89: class com.google.rpc.LocalizedMessage is not a value

on the line:

Trailers(com.google.rpc.Code.INVALID_ARGUMENT, ex.getMessage, List(LocalizedMessage("en-US", ex.getMessage)))

I guess it's possible that gradle isn't configured to know about the scalapb-generated LocalizedMessage. 🤔

[alexklibisz]

@johanandren This is ready for another look. I might need a hint on how to get the Gradle Scala build working.

[johanandren]

The plugin-tester-scala is used for verifying building Scala projects with gradle and maven in addition to the "normal" sbt, so those two project files needed some ammends, I've pushed fixes that should sort it.

[johanandren]

LGTM, thanks! Tricker than you'd think at first.