Labels change (which occurs e.g. when the metrics resolution is changed on the PMM Settings page) was breaking the PostgreSQL dashboards

STR:

1. Run PMM2 Server
2. Add PostgreSQL to monitoring
3. Open PMM Settings page
4. Change resolution to Low
5. Wait for a few minutes
6. Check all PostgreSQL dashboards

Given result: there are errors on charts or doubled lines for the same service
Expected result: There is no such errors on charts