Add playback stats for WebRTC and add test codes

[mekya]

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[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 64.76%. Comparing base (5879368) to head (6b4bb5e).
Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
src/main/js/websocket_adaptor.js	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #506       +/-   ##
===========================================
+ Coverage    7.33%   64.76%   +57.42%     
===========================================
  Files           8        8               
  Lines        1962     1981       +19     
===========================================
+ Hits          144     1283     +1139     
+ Misses       1818      698     -1120     

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