Add total entities metric

[LadyChristina]

All Submissions:

• Have you followed the guidelines in our Contributing documentation?
• Have you verified that there aren't any other open Pull Requests for the same update/change?
• Does the Pull Request pass all tests?

Description

Add total_entities as new metric

New Metric Support Submissions:

• Did you put the metric's script under consensus_decentralization/metrics?
• Did you name the metric's main function of the script compute_{metric name}?
• Did you import the metric's main function to consensus_decentralization/analyze.py?
• Did you add the new metric (and possible parameter values) to config.yaml?
• Did you write unit tests for the new metric?
• Did you document the new metric in the documentation pages?