Refactor duplicate code in Cloud Monitoring backends

[lvaylet]

As highlighted by the # pylint: disable=duplicate-code comments, there is duplicate code in slo_generator/backends/cloud_monitoring.py and slo_generator/backends/cloud_monitoring_mql.py.

Refactor any duplicate code in a separate utils or helpers file, then remove these comments and confirm make lint completes successfully.

[lvaylet]

In addition to utils or helpers, Abstract Base Classes (ABC) are good candidates to rationalize this duplicate code. As described in https://realpython.com/inheritance-composition-python/#abstract-base-classes-in-python:

Abstract base classes exist to be inherited, but never instantiated.

Classes who inherit from an ABC must implement all the methods decorated with @abstractmethod. good_bad_ratio, distribution_cut and query are probably good candidates for such abstract methods, as every backend must implement them. If so, would it make sense to refactor all the backends instead of just the Cloud Monitoring ones?