fix: ignore modifications to unregistered metrics

[sprsquish]

Despite the expiration logic, we were still consuming memory at an alarming rate. Pretty much the same rate as before. If a high cardinality metric that we want to ignore is lumped into a call with a metric we want to keep, we'll update the high cardinality metric. We don't register it, though, so we don't report it. However, it still consumes memory.

We could go at this one of two ways: 1) break up the API calls so we don't request the information from CF and don't update the metric at all; 2) check if we've registered the metric and ignore any call to its update method. This change updates the new metric tracking infrastructure to use option 2.

I also took the opportunity to refactor the Collector implementation for tracked metrics and vibe coded a set of tests (with my own tweaks).