Add jibri_no_instance_failures metric

[bgrozev]

Summary

• Adds a new counter, jibri_no_instance_failures, incremented only when a jibri session fails to start because no instance was available (StartException.NotAvailable).
• Existing per-type failure counters (jibri_sip_failures, jibri_recording_failures, jibri_live_streaming_failures) increment for all failure reasons and cannot isolate this case.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.43%. Comparing base (e98ec08) to head (f03b5cb).

Files with missing lines	Patch %	Lines
...main/java/org/jitsi/jicofo/jibri/JibriSession.java	0.00%	1 Missing and 1 partial ⚠️
...c/main/kotlin/org/jitsi/jicofo/jibri/JibriStats.kt	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1306      +/-   ##
==========================================
- Coverage   51.43%   51.43%   -0.01%     
==========================================
  Files         132      132              
  Lines        8451     8457       +6     
  Branches     1289     1290       +1     
==========================================
+ Hits         4347     4350       +3     
- Misses       3494     3496       +2     
- Partials      610      611       +1     

Files with missing lines	Coverage Δ
...c/main/kotlin/org/jitsi/jicofo/jibri/JibriStats.kt	81.48% <75.00%> (-1.13%)	⬇️
...main/java/org/jitsi/jicofo/jibri/JibriSession.java	38.99% <0.00%> (-0.31%)	⬇️

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