Add mocked tests: get and update database clusters metrics endpoint credentials

[loosla]

Overview

Enhance test coverage by adding mocked test for the function that gets and updates database clusters metrics endpoint credentials.

Mocked tests do not act against the real API so no real resources are created.

To run mocked tests, run: make test-mocked

Instructions

1. Clone this repo.
2. If you haven't already, install Python (version: >= 3.7.2) through their official installer.
3. CD into the directory and run make install
4. Open file: tests/mocked/test_databases.py
5. Add a mocked test for databases.get_cluster_metrics_credentials, databases.update_cluster_metrics_credentials
6. API documentation: get_cluster_metrics_credentials, update_cluster_metrics_credentials

[AryanBakliwal]

Hey @loosla, I've started working on this please assign me