feat(p2p): metrics for ExchangeServer

[Wondertan]

Includes:

• headersServed
• headServeTime
• rangeServeTime
• getServeTime

Depends #125

[codecov-commenter]

Codecov Report

Merging #126 (bb9cfaf) into main (09c272d) will decrease coverage by 2.86%. Report is 1 commits behind head on main. The diff coverage is 29.68%.

@@            Coverage Diff             @@
##             main     #126      +/-   ##
==========================================
- Coverage   67.81%   64.95%   -2.86%     
==========================================
  Files          38       39       +1     
  Lines        3079     3276     +197     
==========================================
+ Hits         2088     2128      +40     
- Misses        843      990     +147     
- Partials      148      158      +10     

Files	Coverage Δ
p2p/helpers.go	78.26% <100.00%> (-2.26%)	:arrow_down:
p2p/peer_stats.go	97.18% <100.00%> (ø)
p2p/peer_tracker.go	78.28% <100.00%> (-0.48%)	:arrow_down:
p2p/session.go	89.79% <100.00%> (+0.10%)	:arrow_up:
p2p/subscriber_metrics.go	7.50% <ø> (ø)
p2p/exchange.go	84.27% <77.77%> (-1.38%)	:arrow_down:
p2p/server.go	83.58% <71.42%> (-1.42%)	:arrow_down:
p2p/server_metrics.go	9.58% <9.58%> (ø)
p2p/exchange_metrics.go	11.30% <11.30%> (ø)