Zap custom jsonpb to allow optional gogo JSONPb serialization

[antoniomo]

Hi,

This is a POC, just to ask if anyone would be interested on it.

Basically it allows me to do:

	// Replace JsonPb logger of zap with the gogo version
	grpc_zap.JsonPbMarshaler = &gateway.JSONPb{}

On my service, so then I can get proper payload logging with gogo types, such as *time.Time when using gogoproto.stdtime = true. In other words, it avoids this bug: gogo/protobuf#212

Since this is go-grpc-middleware, I guess as much compatibility with gogo as possible is desirable?

Let me know if this is interesting and if so, what should I change to make it acceptable to merge (maybe the same for Logrus logger, which would be trivial? Perhaps not import the runtime.JSONPb Marshaler from grpc-gateway to avoid that extra dependency, but have our own version?).

Thanks!

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[antoniomo]

I signed it!

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CLAs look good, thanks!

[codecov-io]

Codecov Report

Merging #149 into master will not change coverage.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master     #149   +/-   ##
=======================================
  Coverage   73.09%   73.09%           
=======================================
  Files          36       36           
  Lines        1349     1349           
=======================================
  Hits          986      986           
  Misses        314      314           
  Partials       49       49

Impacted Files	Coverage Δ
logging/zap/payload_interceptors.go	84.61% <66.66%> (ø)	⬆️

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