Add Mock tests for `PrometheusConnect`

[4n4nd]

Add some mock tests for class PrometheusConnect, so it can be tested without using a real prometheus instance.

I think we can use the mock object library in python for this

[anishasthana]

Would this mock tests be for the purpose of unit testing? Or would it be for an api client user wanting to experiment with the library?

[4n4nd]

Unit testing

[4n4nd]

Oops sorry this should closed