Add basic call metrics

[dennwc]

No description provided.

[codecov]

Codecov Report

Attention: 24 lines in your changes are missing coverage. Please review.

Comparison is base (634148a) 18.51% compared to head (849893d) 21.93%.

Files	Patch %	Lines
pkg/stats/monitor.go	79.22%	15 Missing and 1 partial ⚠️
pkg/sip/inbound.go	37.50%	5 Missing ⚠️
pkg/sip/service.go	72.72%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
+ Coverage   18.51%   21.93%   +3.42%     
==========================================
  Files          19       20       +1     
  Lines        1464     1559      +95     
==========================================
+ Hits          271      342      +71     
- Misses       1167     1189      +22     
- Partials       26       28       +2     

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