Spanner: add 'operation_id' parameter 'Database.update_ddl' (#6737)

[potiuk]

The operation_id im update_ddl method is missing. The parameter is in the documentation https://googleapis.github.io/google-cloud-python/latest/spanner/database-usage.html#update-an-existing-database but it was missing in the implementation.

I've run system tests manually - even if the update_ddl system tests are skipped due to flakyness (I removed the skip annotations and increased timeouts) so I confirm it works :).

Closes #6737.

[tseaver]

Thanks very much for the patch!

Rather than adding another always-skipped system test, could you please add a unit test for the feature in tests/unit/test_database.py?

• Copy test_update_ddl, naming the copy test_update_ddl_w_operation_id.
• In the copy, invoke the method with the operation_id argument.
• Update the assertion at the end to match the passed argument.

[potiuk]

@tseaver: I added the unit test (I have not noticed that those exist for update_ddl in the first place). But I also left the skipped test there, as I found it really useful to test it with real instance. It simply makes it easier - especially that I tried to generate the id based on docs of Spanner and it turned out that the id cannot be too long. Having an example working ID generation in test might be a good hint for someone trying to do the same (if they start to look at the source code).

[tseaver]

@potiuk OK, rather than adding another skipped system test, why don't we just pass the generated operation ID in the existing skipped test?

[potiuk]

@tseaver Sure. Removed the old tests.

[tseaver]

@potiuk Thank you again!