[RTC-310] Prometheus metrics for Jellyfish

[sgfn]

The existing (and new) metric names were changed according to the Prometheus guidelines (see also this page from the telemetry_metrics_prometheus_core docs).

Explanation for the Phoenix metrics can be found here. Personally, I don't really see any use for the metrics which use system_time measurements (they return the system time of when a given event occured, so their graph in time is a linear function ¯_(ツ)_/¯), so I think we should drop them -- but we can discuss this.

EDIT: Right now, as we're subscribed to ICE events, the traffic metrics DO NOT INCLUDE traffic from/to RTSP and HLS components. The Cloud team has agreed to this (right now, they're going to use these metrics as a PoC anyway), but we have to keep this in mind for the future.

[sgfn]

I don't see any usage of those metrics too but Phoenix Team wouldn't include something that is useless, would it?

Cloud says they don't need them, as every metric update in Prometheus comes with a timestamp, anyway. But it doesn't hurt to keep them, I guess

[codecov[bot]]

Codecov Report

Merging #62 (a9156b3) into main (1302b8a) will decrease coverage by 1.89%. Report is 1 commits behind head on main. The diff coverage is 27.77%.

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
- Coverage   86.11%   84.23%   -1.89%     
==========================================
  Files          33       34       +1     
  Lines         504      520      +16     
==========================================
+ Hits          434      438       +4     
- Misses         70       82      +12     

Files Changed	Coverage Δ
lib/jellyfish/application.ex	80.00% <ø> (ø)
lib/jellyfish_web/telemetry/metrics_aggregator.ex	23.52% <23.52%> (ø)
lib/jellyfish_web/telemetry.ex	100.00% <100.00%> (+20.00%)	:arrow_up:

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