Support customization of timestamp format

[stanhu]

By default, RFC3339 timestamps are used, but our application uses a
custom format. This commit enables us to set the format in a consistent
manner.

Closes #131

[johanbrandhorst]

LGTM

[johanbrandhorst]

Thanks for your contribution! Could you please cherry pick this against v2? It may need significant refactoring but it'd be good to have in the new version too.

[stanhu]

@johanbrandhorst Sure, I took a look at the v2 branch. It looks like the new payload interceptors have a bit different signature from logging.interceptors:

go-grpc-middleware/interceptors/logging/payload.go

Lines 126 to 148 in c1f1e53

	// PayloadUnaryServerInterceptor returns a new unary server interceptors that logs the payloads of requests on INFO level.
	// Logger tags will be used from tags context.
	func PayloadUnaryServerInterceptor(logger Logger, decider ServerPayloadLoggingDecider) grpc.UnaryServerInterceptor {
	return interceptors.UnaryServerInterceptor(&payloadReportable{logger: logger, serverDecider: decider})
	}
	// PayloadStreamServerInterceptor returns a new server server interceptors that logs the payloads of requests on INFO level.
	// Logger tags will be used from tags context.
	func PayloadStreamServerInterceptor(logger Logger, decider ServerPayloadLoggingDecider) grpc.StreamServerInterceptor {
	return interceptors.StreamServerInterceptor(&payloadReportable{logger: logger, serverDecider: decider})
	}
	// PayloadUnaryClientInterceptor returns a new unary client interceptor that logs the paylods of requests and responses on INFO level.
	// Logger tags will be used from tags context.
	func PayloadUnaryClientInterceptor(logger Logger, decider ClientPayloadLoggingDecider) grpc.UnaryClientInterceptor {
	return interceptors.UnaryClientInterceptor(&payloadReportable{logger: logger, clientDecider: decider})
	}
	// PayloadStreamClientInterceptor returns a new streaming client interceptor that logs the paylods of requests and responses on INFO level.
	// Logger tags will be used from tags context.
	func PayloadStreamClientInterceptor(logger Logger, decider ClientPayloadLoggingDecider) grpc.StreamClientInterceptor {
	return interceptors.StreamClientInterceptor(&payloadReportable{logger: logger, clientDecider: decider})
	}

It doesn't seem like we can reuse logging.Options here because the Decider and ClientPayloadLoggingDecider signatures are different here. A few options:

1. Introduce a PayloadOptions
2. Tack on the format to the end of each function
3. Expand the Logging interface to set timestamp formatting optoins

[johanbrandhorst]

CC @bwplotka

[stanhu]

I did the simplest thing with option 2 in #399, but I don't love it. Feel free to refactor as necessary.