Initial commit for cpu stats

[hanish520]

Arian wanted to see the amount of CPU and memory saved if some of the replicas chose not to participate in the Handel. I thought maybe it is interesting in general hotstuff experiments to understand the CPU and memory usage. Did not clean up the code, and please let me know if the direction is fine.

[codecov-commenter]

Codecov Report

Merging #63 (035df7d) into master (ce18e82) will decrease coverage by 1.50%. The diff coverage is 2.04%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage   53.08%   51.58%   -1.51%     
==========================================
  Files          70       71       +1     
  Lines        6403     6477      +74     
==========================================
- Hits         3399     3341      -58     
- Misses       2724     2857     +133     
+ Partials      280      279       -1     

Flag	Coverage Δ
unittests	51.58% <2.04%> (-1.51%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
metrics/replicacpumem.go	2.04% <2.04%> (ø)
internal/proto/hotstuffpb/convert.go	76.47% <0.00%> (-16.58%)	:arrow_down:
backend/config.go	48.96% <0.00%> (-15.04%)	:arrow_down:
modules/registry.go	42.85% <0.00%> (-5.59%)	:arrow_down:
backend/server.go	40.74% <0.00%> (-5.56%)	:arrow_down:
consensus/types.go	62.12% <0.00%> (-5.31%)	:arrow_down:
twins/network.go	81.31% <0.00%> (-4.48%)	:arrow_down:
synchronizer/viewduration.go	27.11% <0.00%> (-3.39%)	:arrow_down:
blockchain/blockchain.go	88.77% <0.00%> (-2.05%)	:arrow_down:
crypto/bls12/bls12.go	47.29% <0.00%> (-1.81%)	:arrow_down:
... and 12 more

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[meling]

Could we drop the dependency in favor of the new metrics API in stdlib. See runtime/metrics. Or maybe this makes the whole PR obsolete?