api, metrics: add number of bytes received and sent from/to all entities

[aler9]

Fixes #734

TODO:

• API: number of bytes received/sent from/to RTSP connections
• API: number of bytes received/sent from/to RTSP sessions
• API: number of bytes received/sent from/to RTMP connections
• API: number of bytes sent to HLS connections
• API: number of bytes received from paths
• metrics of all the above

[codecov-commenter]

Codecov Report

Merging #1235 (9d49780) into main (71ef9b4) will decrease coverage by 0.07%.
The diff coverage is 83.63%.

@@            Coverage Diff             @@
##             main    #1235      +/-   ##
==========================================
- Coverage   67.16%   67.09%   -0.08%     
==========================================
  Files         107      107              
  Lines        8941     8934       -7     
==========================================
- Hits         6005     5994      -11     
- Misses       2367     2370       +3     
- Partials      569      570       +1     

Impacted Files	Coverage Δ
internal/core/rtmp_server.go	86.39% <ø> (ø)
internal/core/hls_muxer.go	58.88% <57.89%> (+0.12%)	⬆️
internal/core/metrics.go	87.87% <70.96%> (+3.50%)	⬆️
internal/core/rtsp_session.go	65.23% <92.85%> (ø)
internal/core/hls_server.go	75.74% <100.00%> (ø)
internal/core/path.go	73.09% <100.00%> (+0.24%)	⬆️
internal/core/rtsp_server.go	89.15% <100.00%> (+0.10%)	⬆️
internal/core/stream.go	92.85% <100.00%> (+0.26%)	⬆️
internal/core/streamtrack_h264.go	73.64% <100.00%> (ø)
internal/core/streamtrack_mpeg4audio.go	27.27% <100.00%> (ø)
... and 8 more

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