feat(p2p): metrics for Subscriber

[Wondertan]

Adds 4 metrics:

• messageSize - Histogram
• messageNum (Accepted/Ignored/Rejected) - Int64Counter
• messageTime - Histogram
• subscriptionNum - AsyncGauge

TODO - Graphana Dashboard

[Wondertan]

Btw, @nashqueue, this PR reports sizes for Accepted headers.

[codecov-commenter]

Codecov Report

Merging #119 (300cde6) into main (098c994) will decrease coverage by 1.30%.
Report is 2 commits behind head on main.
The diff coverage is 15.09%.

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
- Coverage   69.20%   67.91%   -1.30%     
==========================================
  Files          37       38       +1     
  Lines        2981     3079      +98     
==========================================
+ Hits         2063     2091      +28     
- Misses        771      841      +70     
  Partials      147      147              

Files	Coverage Δ
p2p/subscription.go	57.14% <75.00%> (+2.97%)	⬆️
sync/metrics.go	15.38% <0.00%> (ø)
p2p/subscriber.go	42.68% <33.33%> (-5.85%)	⬇️
p2p/subscriber_metrics.go	7.50% <7.50%> (ø)

... and 3 files with indirect coverage changes

[walldiss]

Looks good! Some few small cosmetic remarks and optional suggestions.

[walldiss]

🚀