network: add prometheus histogram with cmd processing time

[roman-khimov]

It can be useful to detect some performance issues.

[codecov]

Codecov Report

Merging #2750 (2791127) into master (45f4e8c) will increase coverage by 0.02%.
The diff coverage is 88.46%.

@@            Coverage Diff             @@
##           master    #2750      +/-   ##
==========================================
+ Coverage   85.28%   85.31%   +0.02%     
==========================================
  Files         324      324              
  Lines       40089    40114      +25     
==========================================
+ Hits        34191    34224      +33     
+ Misses       4526     4519       -7     
+ Partials     1372     1371       -1     

Impacted Files	Coverage Δ
pkg/network/prometheus.go	92.50% <85.71%> (-7.50%)	⬇️
pkg/network/server.go	73.00% <100.00%> (+0.10%)	⬆️
pkg/core/transaction/signer.go	75.00% <0.00%> (-12.50%)	⬇️
pkg/core/blockchain.go	81.20% <0.00%> (+0.05%)	⬆️
pkg/network/message.go	94.77% <0.00%> (+0.74%)	⬆️
pkg/core/transaction/transaction.go	86.66% <0.00%> (+1.48%)	⬆️
pkg/network/message_string.go	100.00% <0.00%> (+29.03%)	⬆️

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