I think this is awesome and allows aggregate deltas + descriptor cache to be compatible with collect time filters (resolving #315).

Since both of those features prune their storage via collector usage we should probably add a collector cache TTL that at least matches the largest TTL if either feature is in use. This way if a scrape config changes the collect filters the exporter will eventually clean itself up without needing to be restarted.

Sorry I think I didn't fully explain the purpose of the TTL and how it relates to the descriptor cache/aggregate deltas.

There's three use cases,

1. A Collector is consistently being used -> safe to keep forever
2. A Collector is no longer being used with the descriptor cache and/or aggregate deltas enabled -> Remove after greatest TTL between the two features
   • Both features add memory overhead through caching but prune themselves when the collector is in use
   • In the case of aggregate deltas, this could be a very large amount of memory because every metric is kept around to enable the delta aggregations
   • Removing the collector after the TTL will drop these caches that would otherwise be left around until the exporter is restarted
3. A Collector is no longer being used and descriptor cache/aggregate deltas is disabled -> Remove after the default 2 hour TTL (overhead of these collectors should negligible)

I left some more direct comments for the implementation.