Gitlab::ApplicationRateLimiter declared as class but used as a module

Summary

Gitlab::ApplicationRateLimiter class is declared as class but used as a module only.

Original discussion

The following discussion from !75024 (merged) should be addressed:

• @mkaeppler started a discussion: (+1 comment)

  Question: this confuses me; ApplicationRateLimiter is a class, and it defines an initializer, which suggests I ought to instantiate it, yet we use it as we would use a module that includes self.

  Are there plans to streamline this more in a follow-up? Otherwise I would suggest to fold ApplicationRateLimiter into this module, if we only ever use it like a mixin anyway.