Add protobuf version to ClientInfo instance (i.e. user agent)

[BenRKarl]

Thanks for stopping by to let us know something could be better!

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Is your feature request related to a problem? Please describe.

No, just a feature request.

Describe the solution you'd like

When initializing an instance of ClientInfo, i.e.:

from google.api_core.gapic_v1.client_info import ClientInfo

client_info = ClientInfo()

It should, by default, include the current version of protobuf just like it already does for grpc and other libraries:

client_info.to_user_agent()
>>> "gl-python/3.7.2 grpc/5.5.5 rest/1.1.1 gax/3.3.3 gapic/2.2.2 gccl/1.1.1 pb/4.21.2"

Describe alternatives you've considered

Alternatively we could manually append the protobuf version to the user agent in our library, but it seems cleaner to gather these dependency versions in one location.

[vchudnov-g]

Thanks for the feature request! I replied internally with some additional details. Providing some information on the motivation for this request would also be useful.