metrics: add TransactionGroupTxSyncHandled

[cce]

Summary

Follows #4786 by tallying the total handled TxSyncer messages, to better understand the Remember and AlreadyCommitted counters.

[codecov]

Codecov Report

Merging #4815 (0f36e7a) into master (d80de1a) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #4815      +/-   ##
==========================================
- Coverage   54.67%   54.62%   -0.05%     
==========================================
  Files         417      417              
  Lines       53734    53735       +1     
==========================================
- Hits        29378    29352      -26     
- Misses      21922    21948      +26     
- Partials     2434     2435       +1     

Impacted Files	Coverage Δ
data/txHandler.go	32.55% <0.00%> (-0.20%)	⬇️
network/wsPeer.go	67.06% <0.00%> (-5.02%)	⬇️
ledger/tracker.go	74.04% <0.00%> (-3.83%)	⬇️
ledger/acctupdates.go	69.51% <0.00%> (-0.37%)	⬇️
catchup/service.go	68.96% <0.00%> (ø)
network/wsNetwork.go	66.99% <0.00%> (ø)
crypto/merkletrie/node.go	93.48% <0.00%> (+1.86%)	⬆️
crypto/merkletrie/trie.go	68.61% <0.00%> (+2.18%)	⬆️

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