Include and clarify rds_exporter metrics on Prometheus dashboard to better understand impacts of monitoring RDS instances

The problem: Some charts don't include rds_exporter metrics or metrics has wrong names
e.g.

So that we were unable to understand/see load created by RDS monitoring

Solution:

Formulas have been modified in order to count rds_exporter metrics.

• Scraped Target by Job
• Scrape Time by Job
• Scraped Target by Instance
• Scrape Time by Instance
• Slowest Jobs
• Largest Samples Jobs