Overhaul stats: Add new metric to the metrics endpoint with the current number of banned IPs

[josecelano]

Add a new metric to the metrics endpoint with the current number of banned IPs.

[josecelano]

ACK f7b80ed

[codecov]

Codecov Report

Attention: Patch coverage is 97.05882% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.88%. Comparing base (04de1df) to head (f7b80ed).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
...es/udp-tracker-server/src/banning/event/handler.rs	94.73%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1572      +/-   ##
===========================================
- Coverage    83.93%   83.88%   -0.05%     
===========================================
  Files          284      284              
  Lines        20305    20333      +28     
  Branches     20305    20333      +28     
===========================================
+ Hits         17043    17057      +14     
- Misses        2961     2976      +15     
+ Partials       301      300       -1     

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