feat: Add prometheus metrics.

[bgrozev]

• ref: Move stats config to a separate file.
• ref: Add an abstraction layer in front of statsd.
• ref: Move statsd code under metrics, make private.
• Add a prometheus interface at /metrics.
• feat: Add "healthy" and "recording" metrics.
• ref: Rename a function for clarity.

[codecov]

Codecov Report

Attention: Patch coverage is 6.97674% with 120 lines in your changes are missing coverage. Please review.

Project coverage is 46.34%. Comparing base (081b935) to head (d1a712c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #540      +/-   ##
============================================
- Coverage     47.98%   46.34%   -1.64%     
- Complexity      174      175       +1     
============================================
  Files            73       75       +2     
  Lines          2130     2218      +88     
  Branches        205      198       -7     
============================================
+ Hits           1022     1028       +6     
- Misses         1040     1121      +81     
- Partials         68       69       +1     

Files	Coverage Δ
...main/kotlin/org/jitsi/jibri/metrics/StatsConfig.kt	100.00% <100.00%> (ø)
...n/org/jitsi/jibri/metrics/JibriMetricsContainer.kt	0.00% <0.00%> (ø)
src/main/kotlin/org/jitsi/jibri/Main.kt	0.00% <0.00%> (ø)
...rc/main/kotlin/org/jitsi/jibri/api/xmpp/XmppApi.kt	48.92% <0.00%> (-0.28%)	⬇️
src/main/kotlin/org/jitsi/jibri/JibriManager.kt	5.94% <0.00%> (-2.64%)	⬇️
...rc/main/kotlin/org/jitsi/jibri/api/http/HttpApi.kt	33.87% <0.00%> (-5.02%)	⬇️
...ain/kotlin/org/jitsi/jibri/metrics/JibriMetrics.kt	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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