[Server] Fix Sampling Group creating a new long running task for every Client AddMonitoredItems Method Call

[romanett]

Proposed changes

Fix the SamplingGroup of the Server to only initiate startup of the sampling task if it does not exist yet. As in the previous implementation a new sampling task was started on every Client AddMonitoredItems Service Call for a node being handled by the core Node Manager.

Related Issues

• Fixes Opc.Ua.Server SamplingGroup is spawning too many threads. #2983

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.

Further comments

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 55.43%. Comparing base (cf82847) to head (6bcdb3e).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...braries/Opc.Ua.Server/NodeManager/SamplingGroup.cs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2984      +/-   ##
==========================================
- Coverage   55.50%   55.43%   -0.08%     
==========================================
  Files         352      352              
  Lines       67851    67854       +3     
  Branches    13904    13904              
==========================================
- Hits        37664    37618      -46     
- Misses      26072    26114      +42     
- Partials     4115     4122       +7     

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