Create new instance of client info rather than stomping on shared instance.

[crwilcox]

While reviewing a new client @tseaver noticed a bit that may need revising.

Quoting from https://github.com/GoogleCloudPlatform/google-cloud-python/pull/5665/files/12676aaf08dc795f2216f9441896cfae54edd382#diff-198e257d2c267a0bc3e3d2f3e77ec561

https://github.com/GoogleCloudPlatform/google-cloud-python/blob/master/dlp/google/cloud/dlp_v2/gapic/dlp_service_client.py#L173

        if client_info is None:
            client_info = (
                google.api_core.gapic_v1.client_info.DEFAULT_CLIENT_INFO)
        client_info.gapic_version = _GAPIC_LIBRARY_VERSION

This bit is problematic -- the DEFAULT_CLIENT_INFO instance is going to be scribbled on by every library generated this way, which means that each time an app instantiates a generated client, they all use the last-instantiated one's gapic_version.

Rather than reuse the instance, we should be constructing a new one here, e.g.:

        if client_info is None:  # create a new instance
            client_info = (
                google.api_core.gapic_v1.client_info.ClientInfo(
                    gapic_version=_GAPIC_LIBRARY_VERSION)
        else:  # update caller-supplied instance
            client_info.gapic_version = _GAPIC_LIBRARY_VERSION

[jbolinger]

@crwilcox @lukesneeringer this has been quiet for awhile - is it still relevant?

[tseaver]

@jbolinger The generator is still emitting code which has this flaw (@crwilcox re-genned several libraries over the weekend, ant their clients all have it).

[tseaver]

These snips all scribble on the global DEFAULT_CLIENT_INFO:

• gapic-generator/src/main/resources/com/google/api/codegen/py/main.snip

  Line 140 in 085018f

  client_info.gapic_version = _GAPIC_LIBRARY_VERSION

• gapic-generator/src/test/java/com/google/api/codegen/testdata/py/python_multiple_services.baseline

  Line 964 in 3bcf41a

  client_info.gapic_version = _GAPIC_LIBRARY_VERSION

• gapic-generator/src/test/java/com/google/api/codegen/testdata/py/python_no_path_templates.baseline

  Line 907 in 3bcf41a

  client_info.gapic_version = _GAPIC_LIBRARY_VERSION

• gapic-generator/src/test/java/com/google/api/codegen/testdata/py/python_library.baseline

  Line 1213 in a05cf23

  client_info.gapic_version = _GAPIC_LIBRARY_VERSION