trace dropped packets that are dropped before they are passed to any session

[marten-seemann]

No description provided.

[codecov]

Codecov Report

Merging #2670 into master will decrease coverage by 0.04%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master    #2670      +/-   ##
==========================================
- Coverage   86.76%   86.73%   -0.04%     
==========================================
  Files         124      124              
  Lines        9852     9878      +26     
==========================================
+ Hits         8548     8567      +19     
- Misses        971      977       +6     
- Partials      333      334       +1     

Impacted Files	Coverage Δ
qlog/qlog.go	95.69% <0.00%> (-0.38%)	⬇️
server.go	83.48% <66.67%> (-1.09%)	⬇️
session.go	77.45% <70.59%> (+0.02%)	⬆️
client.go	81.67% <100.00%> (ø)
logging/multiplex.go	95.06% <100.00%> (+0.19%)	⬆️
multiplexer.go	100.00% <100.00%> (ø)
packet_handler_map.go	78.77% <100.00%> (+0.30%)	⬆️

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