feat: track pruned messages in tracer

[twoeths]

Motivation

• In lodestar, metrics showed there are broken promises but if messages are delivered, they are always on time, i.e. within gossipsubIWantFollowupMs
• So I'd like to track messages that are pruned, i.e they're never delivered

Description

• add and track iwantMessagePruned metric

part of ChainSafe/lodestar#4818

[codecov-commenter]

Codecov Report

Base: 83.29% // Head: 83.26% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (2aeccfb) compared to base (23d49be).
Patch coverage: 37.50% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #384      +/-   ##
==========================================
- Coverage   83.29%   83.26%   -0.04%     
==========================================
  Files          48       48              
  Lines       11779    11787       +8     
  Branches     1265     1266       +1     
==========================================
+ Hits         9811     9814       +3     
- Misses       1968     1973       +5     

Impacted Files	Coverage Δ
src/metrics.ts	21.91% <0.00%> (-0.13%)	⬇️
src/tracer.ts	77.45% <75.00%> (-0.06%)	⬇️

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