Accessing commit timestamp after RunTransaction

[radhus]

Is your feature request related to a problem? Please describe.
It's not clear if it's possible to get hold of the commit timestamp when using RunTransaction. It seems like the suggested way is to call the CommitTimestamp function on SpannerConn, but it seems not possible as the RunTransaction controls its own connection.

Describe the solution you'd like
Something useful returned, propagated or otherwise made accessible that can be accessed to read the transaction commit timestamp.

Describe alternatives you've considered
I realize that changing the signature of the function is not feasible, and not sure if additional functions is clear enough. I'm not sure if adding something to the options would make sense either.

Additional context
Coming from the Spanner library, I am used to having the commit timestamp available as a return value from ReadWriteTransaction, but using that library is not wanted or possible in my current project.