feat: type safe metric labels

[nflaig]

See ChainSafe/lodestar#6201 for motivation / rationale

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.37%. Comparing base (eb53ea9) to head (59bd077).
⚠️ Report is 29 commits behind head on master.

Files with missing lines	Patch %	Lines
src/metrics.ts	80.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #479      +/-   ##
==========================================
- Coverage   81.38%   81.37%   -0.02%     
==========================================
  Files          48       48              
  Lines       12275    12261      -14     
  Branches     1292     1292              
==========================================
- Hits         9990     9977      -13     
+ Misses       2285     2284       -1     

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[nflaig]

Label value is not set for this metric

js-libp2p-gossipsub/src/metrics.ts

Line 793 in eb53ea9

this.asyncValidationDelayFromFirstSeenSec.observe((Date.now() - firstSeenTimestampMs) / 1000)

Topic might not be available when setting the metric hence I updated the type instead of updating the code and setting the topic.

[nflaig]

Label was defined in type but not set. I'd assume this was just an oversight as topic is always available here.