[Proposal]: Registry.select_count/2

[Rudolf Manusadzhian]

Registry provides Registry.select/2 as an abstraction on top of `:ets.select/2` to select key, pid, and values using match specs.

In one project we have a potential use case where we would like to only "select_count". 

Registry.count_match/4 doesn't work for us, as it only matches on values under given key, while in our case we want to match on values under any key.

Surely we can first get the list and then count items in it, but that won't be as nice as directly using :ets.select_count/2 

Here I put up together a version that does the same as `Registry.select/2` but uses `:ets.select_count` underneath. 

However, since match spec for "select_count" always assumes the third element to be `[true]` we could, probably only require the pattern and optional guards. Similar to the specs of Registry.count_match/4

Any thoughts?

[José Valim]

Please do send a PR. Although we should probably call it count_select?

Also note that instead of Enum.flat_map you want to use Enum.sum. Make sure to test that scenario too. :)

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[Rudolf Manusadzhian]

Thank you, José! 

I made PR #11405