Revisit the Stats hook to ensure that we're correctly representing operations

[whitfin]

Right now we just track get/set and a couple of other things. It would be better if the Stats hook were modified into tracking all CRUD groups (at least).

Perhaps it would be nice to track the count of results in some form. We could keep track of results on a per-action basis (this is just noted for my memory).

%{
  "clear" => 500,
  "exists?" => %{
    "true" => 10,
    "false" => 20
  },
  "get" => %{
    "ok" => 10,
    "missing" => 20,
    "loaded" => 15
  }
}

Maybe then we could support stats retrieval on a per-command basis which allows us to provide additional info about that stat.

Cachex.stats(:my_cache)                 # very high level stats (op count, etc)
Cachex.stats(:my_cache, :get)           # include things like hit rate using info in the `:get` key
Cachex.stats(:my_cache, [ :get, :set ]) # same as above but for both actions
Cachex.stats(:my_cache, :raw)           # everything we have collected (the struct above)

[whitfin]

Been thinking on this more and I like the above. Although I think there should also be a :global entry which is very general statistics (similar to those tracked currently). Easy to get a high-level overview of your cache usage that way.

If we default to using :global, it has the benefit of keeping the API compatible for anyone relying on the current formats.

[whitfin]

Slight tweaks to the initial notes on how to specify different stat retrieval.

It would become the below instead. This is to avoid any change in the public API, and because it's just an option rather than a requirement.

Cachex.stats(:my_cache, for: :get)