Add Handler method to pass in client

[jacksontj]

Right now if you want to embed the JSON proxy into your service you end
up having to have a client talking to loopback which is messy. With this
API addition you can simply pass in something which implements the
client interface -- meaning you could wrap it yourself and return
directly (without all the loopback and serialization overhead).

[jacksontj]

cc @tmc (seem to be attached to a lot of recent commits)

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[jacksontj]

I signed it!

[googlebot]

CLAs look good, thanks!

[tmc]

Thanks for your contribution! master has a build system fix, can you please rebase this?

[jacksontj]

@tmc done, and I added a commit to regenerate the examples (which makes CI pass)

[achew22]

@tmc this looks good to me. Feel free to rebase and merge

[iluminae]

so this is fantastic and is an obvious advantage over making the TCP call over loopback to achieve the same functionality - but may I bring up that using this bypasses grpc interceptors and stats handlers - which at least some of us use for authentication. (like this guy suggests: http://mycodesmells.com/post/authentication-in-grpc)

Should we open another issue for how to support grpc constructs using this direct call method? Or, is this meant for those not using grpc featurettes like interceptors?

[achew22]

@iluminae brings up a very good point. Could you at a minimum expand upon this downside in the documentation for this new method?

[jacksontj]

@achew22 I've added some documentation clarifying, let me know if that isn't clear enough (and if not, how to change it ;) )

[achew22]

LGTM. once the tests pass let's merge this

[jacksontj]

Forgot to regen the examples, just did-- this run should pass.