Metrics: Don't allocate when invoking the Inc() fastpath

[jannotti]

Almost trivial optimization. No new tests.

[codecov]

Codecov Report

Merging #4193 (f569143) into master (8e845fb) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4193      +/-   ##
==========================================
- Coverage   54.56%   54.55%   -0.01%     
==========================================
  Files         391      391              
  Lines       48788    48788              
==========================================
- Hits        26620    26616       -4     
- Misses      19937    19950      +13     
+ Partials     2231     2222       -9     

Impacted Files	Coverage Δ
crypto/curve25519.go	61.81% <100.00%> (ø)
network/wsPeer.go	65.47% <0.00%> (-3.02%)	⬇️
node/node.go	22.38% <0.00%> (-1.94%)	⬇️
network/wsNetwork.go	64.70% <0.00%> (-0.20%)	⬇️
catchup/service.go	68.88% <0.00%> (+0.74%)	⬆️
data/transactions/verify/txn.go	44.88% <0.00%> (+0.88%)	⬆️
crypto/merkletrie/node.go	93.48% <0.00%> (+1.86%)	⬆️
crypto/merkletrie/trie.go	68.61% <0.00%> (+2.18%)	⬆️
ledger/tracker.go	73.16% <0.00%> (+3.89%)	⬆️

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