[AGENTONB-2764] Refactor SharedMetadata

[swang392]

What does this PR do?

Refactors shared Metadata:

• separated sharedMetadata into sharedMetadata and baseForwarder
• fetch clusterID once at startup, and do not start the metadata forwarder if this step fails (the metadata cannot be used by FA without cluster ID)
• fix inconsistencies between metadata forwarder implementation -- payload method naming, constant organization, logging
• Update tests to include a fakeClient with kube-system namespace to satisfy updated clusterId fetching mechanism

Motivation

What inspired you to submit this pull request?

Additional Notes

Anything else we should know when reviewing?

Minimum Agent Versions

Are there minimum versions of the Datadog Agent and/or Cluster Agent required?

• Agent: vX.Y.Z
• Cluster Agent: vX.Y.Z

Describe your test plan

Noop -- this is a refactor PR so there should be no change in functionality. Tested on abra https://github.com/DataDog/k8s-datadog-agent-ops/pull/7185 and confirmed that metadata for operator, helm and CRDs are being collected.

Checklist

• PR has at least one valid label: bug, enhancement, refactoring, documentation, tooling, and/or dependencies
• PR has a milestone or the qa/skip-qa label
• All commits are signed (see: signing commits)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 58.66667% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.37%. Comparing base (43bd941) to head (2c644c2).

Files with missing lines	Patch %	Lines
cmd/main.go	0.00%	9 Missing ⚠️
pkg/controller/utils/metadata/operator_metadata.go	33.33%	8 Missing ⚠️
pkg/controller/utils/metadata/shared_metadata.go	80.00%	3 Missing and 3 partials ⚠️
pkg/controller/utils/metadata/crd_metadata.go	66.66%	3 Missing and 1 partial ⚠️
pkg/controller/utils/metadata/helm_metadata.go	66.66%	3 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (58.66%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2501      +/-   ##
==========================================
- Coverage   38.38%   38.37%   -0.01%     
==========================================
  Files         300      300              
  Lines       25555    25569      +14     
==========================================
+ Hits         9809     9813       +4     
- Misses      15002    15010       +8     
- Partials      744      746       +2     

Flag	Coverage Δ
unittests	38.37% <58.66%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pkg/controller/utils/metadata/crd_metadata.go	36.41% <66.66%> (+0.32%)	⬆️
pkg/controller/utils/metadata/helm_metadata.go	32.18% <66.66%> (+0.29%)	⬆️
pkg/controller/utils/metadata/shared_metadata.go	86.76% <80.00%> (+0.82%)	⬆️
pkg/controller/utils/metadata/operator_metadata.go	31.00% <33.33%> (-2.34%)	⬇️
cmd/main.go	10.62% <0.00%> (-0.24%)	⬇️

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