Export gapic autogenerated types in the __init__.py of google.cloud.logging_v2

[gkevinzheng]

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Is your feature request related to a problem? Please describe.
PR #804 is a good short-term fix for the issue #582, but we should be considering more long-term fixes.

Describe the solution you'd like
Use the autogenerated __init__.py as a base, which already has the autogenerated types exported in it, then combine it with the handwritten __init__.py.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.
Additional context
Might be related to this issue: googleapis/gapic-generator-python#1642