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

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**Is your feature request related to a problem? Please describe.**
PR googleapis/python-logging#804 is a good short-term fix for the issue googleapis/python-logging#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: https://github.com/googleapis/google-cloud-python/issues/16401