add perfdata to check_pgb_pool checks.

[phoefflin]

changed check_pgb_pool to return pool statistics perfdata for each
checked database.

[df7cb]

Hi,

unfortunately that's not quite it:

POSTGRES_PGB_POOL_SV_USED OK: DB "pgbouncer" (port=6432) pgbouncer=0 * postgres=0 | time=0.04s 'pgbouncer_sv_used'=0; time=0.04s 'postgres_sv_used'=0;