Add $bytes_received to log number of (body) bytes read from client, fixes #892.

[fredrikekre]

NGINX $bytes_received includes header bytes as well, I think (http://nginx.org/en/docs/stream/ngx_stream_core_module.html#var_bytes_received) so perhaps this should be called $body_bytes_receieved? Or perhaps this should be updated to also count header bytes. (NGINX doesn't have $body_bytes_receieved.)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.35%. Comparing base (cf706bc) to head (b2f9e7f).
Report is 176 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #893      +/-   ##
==========================================
+ Coverage   80.12%   80.35%   +0.23%     
==========================================
  Files          36       36              
  Lines        2878     2897      +19     
==========================================
+ Hits         2306     2328      +22     
+ Misses        572      569       -3     

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