fix: [Cherry-pick] Remove time tick delay metrics when nodes go offline (#30833)

[congqixia]

Cherry-pick from master
pr: #30833
See also #30832

This PR removes time tick delay metrics when rootcoord GetMetrics response does not have previously existed querynode/datanode

Also add unit tests for this case

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[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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• internal/rootcoord/OWNERS [congqixia]
• pkg/util/OWNERS [congqixia]

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/run-cpu-e2e

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.18%. Comparing base (e17775a) to head (2113999).
Report is 11 commits behind head on 2.3.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.3   #30879      +/-   ##
==========================================
+ Coverage   82.14%   82.18%   +0.04%     
==========================================
  Files         841      841              
  Lines      121399   121411      +12     
==========================================
+ Hits        99727    99787      +60     
+ Misses      18436    18394      -42     
+ Partials     3236     3230       -6     

Files	Coverage Δ
internal/rootcoord/quota_center.go	90.77% <100.00%> (+2.05%)	⬆️
pkg/util/testutils/prometheus_metric.go	100.00% <100.00%> (ø)

... and 29 files with indirect coverage changes

[weiliu1031]

/lgtm