Release Logging 1.3.0

Add Google::Cloud::Logging::Credentials class.
Rename constructor arguments to project_id and credentials.
(The previous arguments project and keyfile are still supported.)
Document Google::Auth::Credentials as credentials value.
Add partial_success optional argument to Project#write_entries.
Deprecate HttpRequest#method, use HttpRequest#request_method instead.
Update generated low level GAPIC code.
Updated google-gax (grpc, google-protobuf), googleauth dependencies.

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