bazel,grpclb: add a bazel build definition

[jyane]

add a bazel build definition to grpclb.

I think src/generated was created for resolving this issue #357 then I did not add generated files. I just use java_grpc_library.

But Bazel requires a full path from WORKSPACE in a proto file (https://github.com/cgrushko/proto_library/blob/04369f0d2ade8c8566727e0b6f3a53f1ba8925c0/src/person.proto#L5) for importing other proto files.
In this PR, it does not relate, but services will require a full path (master...jyane:add-services-bazel#diff-afc0c0ce1aad649c36404301917e883cR21)

"Do not add generated files" or "Use generated files in Bazel" which is better for now?

[ejona86]

I think src/generated was created for resolving this issue #357 then I did not add generated files. I just use java_grpc_library.

+1. That's what I was hoping to do.

But Bazel requires a full path from WORKSPACE in a proto file for importing other proto files.

Yeah. That's going to be a pain. Just before the holidays @carl-mastrangelo and I were figuring out how we wanted to handle that, because we have that problem internally as well. I think the plan was to generally rely on https://github.com/grpc/grpc-proto for any protos shared between repositories. Those protos could have the proper imports and we'd have copies in the repo for only gradle to use.

We'd still have to figure out something for testing and java-specific protos, since they wouldn't live in grpc-proto. We could maybe just use genrule()+sed to rewrite them.

[ejona86]

nit for in the future: I've been preferring to put the srcs and deps for *_{grpc,proto}_library on one line, since they will normally only have one element and there can be a ton of those rules, for the various languages.

This formatting is still acceptable, though.

[jyane]

I will fix it, please wait for the merging 🙇

[ejona86]

@jyane, I'm going to try to merge #3933 before this PR, to use the shiny new CI.

[jyane]

fixed, PTAL.

Just before the holidays c.arl-mastrangelo and I were figuring out how we wanted to handle that, because we have that problem internally as well.

We'd still have to figure out something for testing and java-specific protos, since they wouldn't live in grpc-proto. We could maybe just use genrule()+sed to rewrite them.

It is very interesting for me :) I have tried to add import_prefix to proto_library but I gave up(jyane/bazel@fc96cc6).

[ejona86]

@zhangkun83, could you also review this?