Add missing nodes stat in miner

[peterlimg]

Fixes

• Update StateMissingNodes stat to display state health in diagnostics pages #2764

Changes

• Add metrics to track the missing state nodes number, time to detect them, and time to sync the missing nodes.

Need to be mentioned in CHANGELOG.md?

Tests

Tasks to complete before merging PR:

• Ensure system tests are passing. If not Run them manually to check for any regressions 📋
• Do any new system tests need added to test this change? do any existing system tests need updated? If so create a PR at 0chain/system_test
• Merge your system tests PR to master AFTER merging this PR

Associated PRs (Link as appropriate):

• blobber:
• gosdk:
• system_test:
• zboxcli:
• zwalletcli:
• Other: ...

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (sprint-1.11@b1b876e). Click here to learn what that means.
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@@              Coverage Diff               @@
##             sprint-1.11    #2813   +/-   ##
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  Hits                   ?    18869           
  Misses                 ?    44862           
  Partials               ?     2273           

Flag	Coverage Δ
Unit-Tests	28.58% <0.00%> (?)

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[dabasov]

lgtm