[BE] Amazon RDS PostgreSQL monitoring

User story:
As PMM Admin user I need to be able to use the Discovery feature and add to the Monitoring AWS based PostgreSQL instances so I can monitor their performance

UI/UX:
N/A, same as existed

Acceptance criteria

• User needs to be able to provide AWS credentials and see all list of supported DB, including Postgresql related of all versions (Discovery) 
• User able to proceed to add RDS PG instances without  filling all fields needed for rds monitoring  (same as for MySQL)
• User able to use API to add RDS PG instances to the monitoring  
• need to be tested with: 
  • AWS RDS PostgreSQL - Versions: 11.10, 12.5 (latest )
  • AWS Aurora ( PostgreSQL)  - Versions: 11.9, 12.4 (latest)

Out of scope:

• TBD

Suggested implementation:
TBD

How to test:
TBD

Details:

Currently the RDS exporter is only being used when adding MySQL RDS instances.

 However, there's a workaround to collect Postgres RDS cloudwatch metrics with the rds_exporter bundled with PMM: 

• Add the PG RDS instance normally with the "Add a remote instance" button from the PMM Add Instance Dashboard
• Once added, find out the node id, for example: /node_id/7c65ee37-5b15-4632-862a-af1dba2452c3
• Use the API method /v1/inventory/Agents/AddRDSExporter. Pass the node id, and the access/secret aws keys
• Agent will be added

Now, here's the workaround: The configuration file created inside the container in /tmp/rds_exporter/pmm-server/rds/config needs a tweak. It will add as "instance" the eds endpoint, not the db identifier:

- region: us-west-2
 instance: my-rds-instance.xxxxxxxx.us-west-2.rds.amazonaws.com <---HERE
 aws_access_key: XXXXXXX
 aws_secret_key: XXXXXXXX
 disable_basic_metrics: false
 disable_enhanced_metrics: false
 labels:
 agent_id: /agent_id/cdc91933-b973-4414-a2cc-e5462541dbaa
 agent_type: rds_exporter
 node_id: /node_id/69f2d5a0-e518-44f7-b6b1-530bcc62f0f5
 node_model: db.m5.xlarge
 node_type: remote

The fix is to modify the "instance" by changing the endpoint to the actual db identifier (the part before the first dot) and force the rds_exporter process to be restarted (i do it with a kill -9 <pid>)

Once that is done, CloudWatch metrics starts to become collected

So, it will only takes to fix the agent code to use the db instance name instead of the endpoint to make it work with Postgres RDS