Fix lint warnings and failing unit tests

Update cos_agent charmlib to v0.5

Only start mysqld-exporter on cos relation created and stop it on relation broken

Expect mysqld-exporter to be disabled by default for replication integration tests

Add missing await statements in exporter integration test

Use retry to determine if mysqld-exporeter service has started

Set monitoring username and password for exporter in integration test

Fix how the username and password were being set for the exporter in the integration test

Run format

Sleep to wait until mysqld-exporter started

Run format

Address PR feedback

Address PR feedback