Set gRPC user-agent when calling google APIs

[aabmass]

Fixes googleapis/python-api-core#63

Adds user-agent to gRPC calls. Note this will not work if the user overrides the client on their own.

Added an integration test which checks the actual user-agent metadata sent in gRPC requests

[punya]

Should we try to simplify this change by proposing changes to make gax and/or python-monitoring more configurable? Overriding user-agent seems like a common enough use case that they shouldn't make it this hard.

[aabmass]

I dropped a comment here googleapis/google-cloud-python#15081 (comment). What are your thoughts here in terms of submitting this vs pushing on upstream libraries @punya?

[punya]

I'd like us to start by proposing the upstream change that we'd ideally want. If we encounter resistance, we should fall back to this approach.

@jsuereth and @dashpole, curious to hear your thoughts too.

[dashpole]

I'd prefer the reverse. Make it work here, and open issues (here and upstream, if one doesn't already exist) to track making it cleaner/easier upstream.

[aabmass]

@punya @dashpole can we go ahead with this PR?

[punya]

LGTM