low priority feature req - automatically remove stale flags

[ghost]

keep track of uses of a given flag using simple metric system + automatically delete/remove flags that haven't been referenced after a stale interval

[tompave]

I see how this could be useful, but I don't think that it should be added to FWF as a feature. This is because of a few reasons:

1. FWF aims to be simple, and do one thing well.
2. Adding a usage-tracking system and increment some counter (writing data somewhere) every time a flag is queried is going to have a performance overhead. This needs to be understood before adding something of this sort. I wonder if this could be done in-memory, without counter persistence (therefore, with less overhead), but then the usage tracking would become unreliable.
3. This is something that can be easily added in application code with a wrapper for FWF, and even released as an extension on Hex. For example, your application could use a facade module that wraps FWF and its enabled? function, increments the counter with each call, and then runs some sweeper GenServer that periodically deletes stale flags.