Metric still returned for excluded databases

[KES777]

See details here: #713

Metric still returned for excluded databases:
http://172.22.22.15:9187/probe?target=tsdb:5432&auth_module=tsdb

...
pg_database_size_bytes{datname="postgres"} 1.641923363e+09
pg_database_size_bytes{datname="template0"} 8.602115e+06
pg_database_size_bytes{datname="template1"} 1.1125251e+07
...