fix: client version missing from the user agent header

[plamut]

Fixes #200.

This PR patches the generated code so that client library version is actually included in the x-goog-api-client metrics key.

PR checlist:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[tswast]

Is this something we can update in the Bazel config to avoid manual overrides? https://github.com/googleapis/googleapis/blob/master/google/pubsub/v1/BUILD.bazel

[anguillanneuf]

Not quite familiar with BUILD.bazel, but it looks like pubsub should belong in https://github.com/googleapis/googleapis/tree/master/google/cloud just as pubsublite?