Overhaul stats: Use broadcast channel for events in HTTP Core

[josecelano]

Stats events were introduced to collect tracker metrics. We only have global metrics (aggregate metrics for all UDP and HTTP trackers).

This will change in the future. We will have:

• Segregated metrics: one listener per tracker (per socket).
• Generic events: there could be other events consumers. Events will be decoupled from stats.

This change allows multiple receivers in the channel.

For now, we one use one listener but with this change will be easy to add more.

[josecelano]

ACK 5f9c4d3

[codecov]

Codecov Report

Attention: Patch coverage is 80.55556% with 7 lines in your changes missing coverage. Please review.

Project coverage is 81.88%. Comparing base (3ee074f) to head (5f9c4d3).
Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ages/http-tracker-core/src/statistics/event/mod.rs	0.00%	6 Missing ⚠️
...http-tracker-core/src/statistics/event/listener.rs	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1391      +/-   ##
===========================================
- Coverage    81.90%   81.88%   -0.02%     
===========================================
  Files          232      232              
  Lines        17091    17081      -10     
  Branches     17091    17081      -10     
===========================================
- Hits         13998    13987      -11     
+ Misses        2842     2841       -1     
- Partials       251      253       +2     

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