Make CloudSpanner log implementation GA

[AlCutter]

No description provided.

[AlCutter]

Need to document a list of blockers

[RJPercival]

While working on #1772, I came across this graph for Cloud Spanner which indicates lots of already_exists errors for Commit requests and failed_precondition errors for Rollback requests in our CI environment. Presumably this is normal, since the deployment appears to be operating successfully, but I was wondering whether we could do anything to reduce the number of failed_precondition errors since this seems like something we'd otherwise want to alert on?

[AlCutter]

Just had a dig into it - it's due to the fact that the CT hammer creates a lot of duplicate submissions.

I've filed #569 to track that, and popped a quick hack into the version of hammer running against spanner CI to disable dupes for the moment.

The linked graph has dropped off to zero now.

[RJPercival]

I'm guessing the rollback errors were because we were calling Rollback after Commit when receiving already_exists, which isn't allowed? Thanks for looking into that.