[BEAM-53] Pub/sub client with gRPC implementation

[mshields822]

Support pub/sub via gRPC.
Will later use for pure Java pub/sub source/sink.

[mshields822]

R: @kennknowles

[kennknowles]

I think the official name is "Pubsub", no slash, capital P, lowercase 's'.

[mshields822]

Done.

[kennknowles]

close paren

[mshields822]

Done.

[kennknowles]

Looks like checkstyle dislikes your copyright headers.

[mshields822]

Could do with a squash but this addresses your comments and supports some quirks needed later by the dataflow runner.

[mshields822]

Looking at the misc support code in PubsubIO thinks would be much clearer if we also had a PubsubApiaryClient and all the regex and other misc handling in PubsubIO was moved into PubsubClient.

BUT I'd like to make steady progress, so how about we take that as a TODO?

[kennknowles]

LGTM. Merging.

[kennknowles]

It occurs to me that this PR would really benefit from a follow-up with an in-memory testing fake, with a test suite that can be applied to both it and the gRPC implementation.

[davorbonaci]

I think this PR might not have received the attention it deserves:

• We are adding new dependencies. Have we considered the impact on shading and leakages? Any impact on the dependency tree? How about transitive dependencies?
• We are adding new public APIs in the io package. Perhaps it would be more appropriate for the util package, or package-visible API.
• We seem to be missing tests for these new APIs.
• We seem to be importing repackaged Preconditions. Is there a reason to?
• Etc.

Since this is IO-related, @dhalperi would be a really good reviewer. @mshields822, can I ask you to get these things fixed up?

[kennknowles]

Whoops

[kennknowles]

Package-private would be better than util/, and it does seem possible for now at least.

[mshields822]

No worries:

1. Flink can't handle gRPC without special magic (openSSL dep problems), so that's the last straw for me with gRCP. I'm going to add a PubsubApiaryClient and see what I can do to bring the gRPC deps back under control.
2. Only reason any of this is public is so the Nexmark driver can piggy back on it for all its pubsub create/list/delete stuff. Davor suggested util/ is a happy place for this sort of code, will follow that.
3. Repackaged slipped through an auto-complete. Sorry about that.
4. Hard to unit test this and I didn't want to go to town on integration or service tests, especially since it is much easier to test at the pipeline level than the raw client level.
   Stand by.

[mshields822]

Also, the extra deps (beyond grpc-pubsub-v1) are all needed by the gRPC client. They are also included by the grpc-pubsub-v1 dep, but our transitive dependency check requires them to be included directly. You can find the same deps inside the bigtable-client-core pom. So the only new dep here is on grpc-pubsub-v1 itself.