Add row count metric for each table

[joshua-spacetime]

From @kurtismullins:

There are currently metrics around the time it takes to do various operations to tables. To shine a little context on these operations, lets expose a metric which simply states how many rows a given table has.

If this already exists, feel free to just let me know and close this issue. I couldn't find it but may have missed it. An example metric which would be more useful if we knew the row count is: spacetime_rdb_iter_time_count.

[joshua-spacetime]

Now that we have metrics for inserted and deleted rows

spacetime_num_rows_inserted_cumulative
spacetime_num_rows_deleted_cumulative

This metric can be derived as the difference between the two.

[joshua-spacetime]

Actually it would be better for this to be its own metric (gauge) because inserts - deletes will not be accurate unless starting from 0.