Add Rubocop rule to make sure we inherit only allowed metric classes

What does this MR do?

Related to #329103 (closed)

This adds a Rubocop cop that checks whether metric instrumentation classes subclass one of the base metric classes:

• DatabaseMetric
• GenericMetric
• RedisHLLMetric

Based on Rubocop::Cop::EnforceSuperclass from rubocop-rails.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.