[postgres] include table count as a metric

[stefanfoulis]

Amount of tables per database.

Possibly the instructions on how to prepare postgres for datadog would need to be changed, since currently only access to the pg_stat_database view is granted and the information is not in there.

[LeoCavaille]

@stefanfoulis I guess you would like to see info about user tables and not all tables (including toast tables), would something like converting this request to metrics make sense to you ?

SELECT count(*), schemaname FROM pg_stat_user_tables GROUP BY schemaname;

[stefanfoulis]

@LeoCavaille yep, that looks about right :-)

[remh]

Will be done by #1373

postgresql.total_tables

[remh]

PR is merged, this will go out with 5.3.0.