[Server] Fix memory leak of Monitored Items in CustomNodeManager2

[romanett]

Proposed changes

Fixes a Memory leak of Monitored Items introduced in 1.5.377 with #3075

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.91%. Comparing base (e03883a) to head (8fcb836).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3324      +/-   ##
==========================================
- Coverage   60.24%   58.91%   -1.34%     
==========================================
  Files         323      323              
  Lines       76121    76122       +1     
  Branches    13361    13361              
==========================================
- Hits        45862    44846    -1016     
- Misses      26112    27175    +1063     
+ Partials     4147     4101      -46     

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