Add RDS tags to all rds exporter metrics

[dianamelinte23]

Is your feature request related to a problem? Please describe

Currently, RDS instance tags are only available in the rds_instance_tags metric. However, they are not attached to other metrics such as: rds_instance_max_throughput_bytes, rds_instance_status, etc. This makes it difficult to filter or group metrics by logical application identifiers (e.g., team, service, environment) using tags — especially in Prometheus or Grafana — without complex recording rules.

Describe the solution you'd like

We would also like to segregate dashboards per team in Grafana, with each team viewing only metrics tagged with their application code under their own Grafana organization, therefore we need all RDS tags attached to all exported metrics.
Optionally provide an easy way to filter metrics by tag in exporter — for example, an include-tags-on-metrics: true option or global whitelist of tag keys to include in all metrics.

[vmercierfr]

Having all AWS tags attached to each Prometheus metrics will increase Prometheus metric cardinality, storage and memory.
If any tag is changed, it will create new metrics in Prometheus.

A more efficient way to achieve this is to leverage the Prometheus features to join metrics.

Did you considered using selectors (aka Grafana dashboard variable) to select the team/environment and choose the instance?

For instance:

label_values(rds_instance_tags,tag_project)	
label_values(rds_instance_tags{tag_project="$project"},dbidentifier)