Add generator for new metric instrumentation classes

What does this MR do?

Related to #326420 (closed).

This adds a Rails generator for creating the new Usage Ping instrumentation classes along with spec stubs. Example usage is described in the USAGE banner file.

I excluded RSpec/TopLevelDescribePath from fixture files, because the cop tries to match the spec template like a legitimate spec. On the other hand, if the file ends with *_spec.rb, then RSpec tries to load it.

Does this MR meet the acceptance criteria?

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