fix: only stop task when every clone of it has been dropped

[marsavar]

Thanks @chmodas for this crate!

This fixes an issue where the shutdown flag gets permanently flipped to true (which signals that the polling task should be stopped) any time a cloned pool is dropped. Because all the pool clones share the same Arc<AtomicBool>, the first clone to be dropped incorrectly flips the flag, which is not what the documentation implies.

This also adds a regression test to make sure the metric task surives a cloned pool being dropped

[chmodas]

Hi @marsavar , thanks for the fix! Apologies for the delay in getting back to you – life got in the way a bit. I’m going to merge this and push out a new release now.

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✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.96%. Comparing base (2772244) to head (351d433).
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Additional details and impacted files

@@           Coverage Diff           @@
##            trunk      #36   +/-   ##
=======================================
  Coverage   99.96%   99.96%           
=======================================
  Files          13       13           
  Lines        2552     2555    +3     
=======================================
+ Hits         2551     2554    +3     
  Misses          1        1           

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