Consider deduplicating Metrics processor strings and ES client class descriptions

[mlw]

    Something to follow-up on (possibly later) - these strings are returned by the `description` method in each of the event provider classes. We should find a way to use the class name instead of having this strings added here too, and potentially remove the Processor enum.

Originally posted by @russellhancox in https://github.com/google/santa/pull/949#discussion_r1041455427