Add `GrpcCircuitBreakerRule`

If a CircuitBreakerRule is built for gRPC clients, users might want to get grpc-status to decide whether a call is a success or not. Normally, grpc-status exists in the HTTP trailers of a response. However, if a call is failed, it could be in the HTTP headers of a trailers-only response. It is a transport-level protocol specification and many users may not know the details.

If we add GrpcCircuitBreakerRule which inherits the existing CircuitBreakerRule and add onGrpcStatus(), users will be able to use circuit breaker with gRPC clients more easily.

[ikhoon]

it could be even better if the GrpcCircuitBreakerRule has a method onGrpcTrailers(grpcTrailers: HttpHeaders)

Agreed! In addition to onGrpcStatus(), onGrpcTrailers() would be a good extension method for GrpcCircuitBreakerRule and GrpcRetryRule.

[Lincong]

Generally I think adding GrpcCircuitBreakerRule with a onGrpcStatus method is a great idea.

However, it could be even better if the GrpcCircuitBreakerRule has a method onGrpcTrailers(grpcTrailers: HttpHeaders) where grpcTrailers is “trailers” in the context of gRPC. Specifically, grpcTrailers could be either HTTP headers if the response is “trailers-only” or HTTP trailers if the response has 3 parts (HTTP headers, body, and HTTP trailers).

This onGrpcTrailers(grpcTrailers: HttpHeaders) is more generally useful because users get access to the whole gRPC trailer which must contain gRPC status (as promised by the gRPC protocol specification). Another way to think about it would be: what if users want to set up their circuit-break conditions upon other information stored in gRPC trailers other than just gRPC status?

Let me know what you think. Thanks!