Add gRPC support

[jhalterman]

Add gRPC support as a middleware, similar to the RoundTripper based HTTP support.

[szabba]

This should actually work with the gRPC-compattible connect-go already.

[proost]

I need this feature. could i contribute this?

[jhalterman]

Sure - I could review a contribution for this. The failsafehttp package, for comparison, allows you to create either a Failsafe RoundTripper, or a Failsafe Request. It also has a RetryPolicyBuilder for building retry policies that handle common response codes.

I don't have any ideas yet for what a failsafegrpc package might look like, but I'm interested to hear what you think.

[proost]

I plan to use UnaryClientInterceptor and StreamClientInterceptor.

Same as failsafehttp, supporting RetryPolicyBuilder to handle common retryable grpc status code(ex. DEADLINE_EXCEEDED, UNAVAILABLE) is good.

[jhalterman]

That's sounds good. On the server side, we should also allow creating a failsafe tap handle (used via grpc.InTapHandle) that could be used for things like server side circuit breakers or rate limiters.