use *Server interfaces in implementations of fake*Server

[Capstan]

In equivalence_test.go, there's a fake implementation of MetricService. It implements exactly the methods of that service as of now. Thus, if new methods are introduced (or in the case of the mono-repo, there are additional internal-only methods), this causes a compilation break, even if those methods are never exercised.

The protocol compiler will produce an UnimplementedMetricServiceServer which provides dummy methods that return NYI errors. If this is used as a field of the fake, e.g.,

type fakeMetricsServer struct {
  monitoringpb.UnimplementedMetricServiceServer
  …

then if methods are introduced, the fake will still compile and the new methods will just return errors. In this particular case, all the methods unused by the test could just be removed, since it will still fulfill the interface.

[Capstan]

Appears that this isn't rendered as a part of genproto, so perhaps use of just the raw MetricServiceServer interface would do. It won't nicely error, but just panic if a nil method is called.