metrics: TagCounter gains a tag set to filter on

[brianolson]

Summary

unknown tags can be counted under one substitute tag e.g. "UNK"
limit protocol counters to known protocol tags

Test Plan

Added unit test.

[codecov]

Codecov Report

Merging #4503 (96aeba2) into master (e83acec) will decrease coverage by 0.03%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master    #4503      +/-   ##
==========================================
- Coverage   55.25%   55.22%   -0.04%     
==========================================
  Files         398      398              
  Lines       50297    50310      +13     
==========================================
- Hits        27793    27784       -9     
- Misses      20171    20192      +21     
- Partials     2333     2334       +1     

Impacted Files	Coverage Δ
util/metrics/tagcounter.go	82.27% <81.81%> (-0.08%)	⬇️
network/wsPeer.go	65.66% <100.00%> (-2.83%)	⬇️
ledger/roundlru.go	90.56% <0.00%> (-5.67%)	⬇️
ledger/blockqueue.go	85.63% <0.00%> (-2.88%)	⬇️
catchup/peerSelector.go	98.95% <0.00%> (-1.05%)	⬇️
cmd/tealdbg/debugger.go	72.69% <0.00%> (-0.81%)	⬇️
ledger/acctupdates.go	69.59% <0.00%> (-0.60%)	⬇️
network/wsNetwork.go	64.63% <0.00%> (-0.20%)	⬇️
catchup/service.go	68.88% <0.00%> (ø)
... and 3 more

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[cce]

Great!

[winder]

LGTM