[ENG-2343] Stream attempts metric

[pwilczynskiclearcode]

What does this pull request do? Explain your changes. (required)
A new prometheus counter added - ai_live_attempt. The metric counts the number of AI live stream start attempts. The metric is generated early, before any connection to Os.

We could additionally add a runOnConnect mediamtx hook sth like:

MTX_PATHDEFAULTS_RUNONCONNECT: curl "..../live/video-to-video/$MTX_PATH/connect"

receive it on the AI Gateway and log to prometheus from there

[linear]

ENG-2343 Metrics: Add "stream attempts" metric

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 33.73146%. Comparing base (b026313) to head (9d4f9fe).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
server/ai_http.go	0.00000%	3 Missing ⚠️
server/ai_process.go	0.00000%	3 Missing ⚠️
monitor/census.go	80.00000%	2 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3347         +/-   ##
===================================================
+ Coverage   33.72537%   33.73146%   +0.00609%     
===================================================
  Files            141         141                 
  Lines          37411       37425         +14     
===================================================
+ Hits           12617       12624          +7     
- Misses         24073       24080          +7     
  Partials         721         721                 

Files with missing lines	Coverage Δ
monitor/census.go	62.10526% <80.00000%> (+0.11851%)	⬆️
server/ai_http.go	9.98440% <0.00000%> (-0.03125%)	⬇️
server/ai_process.go	0.59778% <0.00000%> (-0.00102%)	⬇️

... and 1 file with indirect coverage changes

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Files with missing lines	Coverage Δ
monitor/census.go	62.10526% <80.00000%> (+0.11851%)	⬆️
server/ai_http.go	9.98440% <0.00000%> (-0.03125%)	⬇️
server/ai_process.go	0.59778% <0.00000%> (-0.00102%)	⬇️

... and 1 file with indirect coverage changes