Fix MetricReceivedProposedBlock overflow

[iulianpascalau]

Reasoning behind the pull request

• there were situations when the MetricReceivedProposedBlock was improperly computed

Proposed changes

• fix MetricReceivedProposedBlock overflow by disallowing a negative value

Testing procedure

• standard system test + system test without transactions

Pre-requisites

Based on the Contributing Guidelines the PR author and the reviewers must check the following requirements are met:

• was the PR targeted to the correct branch?
• if this is a larger feature that probably needs more than one PR, is there a feat branch created?
• if this is a feat branch merging, do all satellite projects have a proper tag inside go.mod?

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (rc/v1.4.0@a3ca9d0). Click here to learn what that means.
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##             rc/v1.4.0    #4831   +/-   ##
============================================
  Coverage             ?   70.74%           
============================================
  Files                ?      647           
  Lines                ?    85115           
  Branches             ?        0           
============================================
  Hits                 ?    60214           
  Misses               ?    20368           
  Partials             ?     4533           

Impacted Files	Coverage Δ
consensus/spos/worker.go	82.08% <100.00%> (ø)

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[gabi-vuls]

System test passed