fix: Handle interceptor errors as responses (#840)

[NAlexPear]

Summary

This PR forwards Interceptor errors/Statuses as valid HTTP responses, just like any other gRPC service generated by tonic.

Motivation

In the very specific case of missing Content-Type headers when requests are intercepted, this PR fixes the bugs reported in #840 and #759.

In the more general case, this class of bug arose because interceptors and their statuses are treated differently from the rest of the gRPC service stack: their errors/Status are bubbled up as errors to be handled at the tower service layer, but not as a gRPC response. That means that a basic HTTP response is returned, sans trailers or headers of any kind (including the missing Content-Type header). See this repository and this comment to compare the responses generated by returning a Status from a method versus an interceptor.

This behavior means that clients like grpcurl that encounter the responses from interceptors don't know how to handle them, and it's likely that this affects any consumer that depends on the Content-Type header (among others).

Solution

The solution in this PR is to handle Status errors the same way in both interceptors and regular ol' gRPC services: by converting those statuses to valid responses in the Interceptor service itself, matching the behavior in the existing grpc module.

The downside to this approach is a few additional constraints on the response body type. These constraints seem reasonable given that they match up with the current uses of interceptors in tonic, but they do make wrapping non-tonic services trickier.

Next Steps

EDIT: looks like those constraints are too restrictive for tonic-health 🤦. I'll take a look at either relaxing those constraints or updating tonic-health accordingly.

EDIT 2: I've both relaxed the constraints and added a bit of restriction to clients in tonic-build. I'm not sure what the right mix between the two should be for a final implementation, but this does solve the issue for my use-case.

[LucioFranco]

LGTM!

[okkero]

Looking forward to seeing this merged

[olliebatch]

Looking forward to seeing this merged, Prevents the use of grpcCurl as it errors so will be useful!