Allow restricting exporter scope to specific instances

[mindw]

Is your feature request related to a problem? Please describe.

I'm looking for exporting the output of describe-db-instances to prometheus. This promising project seems to fit the bill - except it returns metrics for all RDS instances within region. At least I could not an example on how to do it.

Describe the solution you'd like

Add an option to restrict exporter to an RDS instance(s) by - id, regex, tag (key/*, key/val, key/regex)

Describe alternatives you've considered

Use the archived https://github.com/movio/aws-prometheus-exporter exporter to expose describe-db-instances output.

Thanks!