tests: fix network metrics race condition in tests

[algorandskiy]

Summary

There was a race condition between TagCounter.Add and TagCounter.WriteMetric when accessing the counter.

Test Plan

The issue discovered by a new test, no new tests needed.

[codecov]

Codecov Report

Merging #4529 (5c064c0) into master (22b6101) will decrease coverage by 0.05%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #4529      +/-   ##
==========================================
- Coverage   54.06%   54.01%   -0.06%     
==========================================
  Files         401      401              
  Lines       51540    51542       +2     
==========================================
- Hits        27863    27838      -25     
- Misses      21346    21368      +22     
- Partials     2331     2336       +5     

Impacted Files	Coverage Δ
util/metrics/tagcounter.go	81.48% <50.00%> (-0.80%)	⬇️
ledger/roundlru.go	90.56% <0.00%> (-5.67%)	⬇️
util/db/dbutil.go	44.24% <0.00%> (-4.25%)	⬇️
ledger/blockqueue.go	85.63% <0.00%> (-2.88%)	⬇️
network/wsPeer.go	66.03% <0.00%> (-2.16%)	⬇️
catchup/peerSelector.go	98.95% <0.00%> (-1.05%)	⬇️
ledger/acctupdates.go	69.59% <0.00%> (-0.60%)	⬇️
catchup/service.go	69.38% <0.00%> (+0.74%)	⬆️

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