Add Req/Res count/time to candidate stats

[cnderrauber]

See pion/ice#763

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.32%. Comparing base (44062a7) to head (490f99b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3043      +/-   ##
==========================================
- Coverage   78.41%   78.32%   -0.10%     
==========================================
  Files          91       91              
  Lines       11251    11254       +3     
==========================================
- Hits         8823     8815       -8     
- Misses       1938     1946       +8     
- Partials      490      493       +3     

Flag	Coverage Δ
go	80.19% <100.00%> (-0.11%)	⬇️
wasm	63.39% <0.00%> (-0.05%)	⬇️

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