Add request_context flag to utilize (*http.Request).Context() in handlers

[tmc]

This is a simplification of #251.

This PR adds the request_context flag which when true will utilize the incoming *http.Request's context when invoking grpc methods.

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[tmc]

@kokaz can you comment here to pass the cla bot?

[zirkome]

I agreed to the CLA @googlebot

[tmc]

Thanks @kokaz -- I want one more LGTM from another contributor before merging. ping @yugui @kazegusuri @achew22 @t-yuki

[achew22]

Is there a strategy for making this the default in the future or will it remain an optional feature?

Should we have a generated example with this turned on?

[tmc]

I think this should become the default once we're okay with degrading 1.6 support.
I added testing this code path to the travisci configuration, I'm not certain adding a generated example provides that much value.

[zirkome]

@tmc Is there any progress on that?

[AmandaCameron]

Any chance on getting this merged sometime soon? It'd be very helpful with stuff such as routing tracing through the grpc-gateway to the backend grpc service.

[achew22]

@tmc what information are you trying to get out of the HTTP request context that isn't available now?

[tmc]

@achew22 i think a pretty good use-case is tracing (opentracing or google cloud trace)

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[AmandaCameron]

Tracing is what my usecase is for this PR. It's easier to thread the nessary tracing information through a http middleware instead of a grpc serverinterceptor, and more clean, I feel. It'd also allow middleware to do whatever other context manupulation it wants.

[yugui]

LGTM with a comment -- I expect that this behavior will be enabled by default at some point because I did not do this just because req did not have Context().

I really love this PR and I believe this is the direction we should take.