Allow filtering out metrics for pools with no connections

[maartenberg]

This PR adds a new config option filter_empty_pools. When enabled, this option causes the exporter to not emit pool metrics for pools with no active connections (all values zero).

This filtering is useful when you use PgBouncer with temporary database users: currently PgBouncer will never "forget" a user and keep reporting an empty pool for each user it has ever seen (pgbouncer/pgbouncer#1085).
Enabling this filtering speeds up metric collection and (for us) greatly reduces the amount of emitted metrics.

The new config option is added to the README and has a unit test to verify it works as described.

This PR also contains two other changes:

• Fixes a SyntaxWarning related to comparing strings using is by using == instead.
• Slightly speeds up metrics filtering by making _filterMetricsByIncludeDatabases and _filterMetricsByExcludeDatabases return generators instead of lists.
  This way the exporter needs to loop over the results only once (when actually generating metrics), instead of once for every filter that's enabled.