[java-spanner-jdbc] Get the size of the database

**Is your feature request related to a problem? Please describe.**
I'd like to be able to determine the size of a Spanner database.

**Describe the solution you'd like**
Some kind of API in the JDBC driver that returns the size of the database.

**Describe alternatives you've considered**
We've tried querying the information schema to get the list of tables, then trying to figure out the data usage of each field in every table. However this is not performant, and error prone.

**Additional context**
n/a