allow forwarder to be used by all controllers

[celenechang]

What does this PR do?

• Define metricsForwarder one level up, outside of the DatadogAgent controller, so it can be used by other controllers
• Allow metricsForwarder to use credentials configured in the Operator

Motivation

Make metricsForwarder more generic

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

Make sure that metrics forwarder works as expected (datadog.operator metrics are received), under two scenarios:

• Credentials are not set in the Datadog Operator deployment, but are set in the DatadogAgent CR
• Credentials are set in both the Datadog Operator deployment and in the DatadogAgent CR

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 73.91304% with 6 lines in your changes missing coverage. Please review.

Project coverage is 48.64%. Comparing base (b70d772) to head (9556e4c).

Files with missing lines	Patch %	Lines
internal/controller/setup.go	63.63%	2 Missing and 2 partials ⚠️
internal/controller/datadogagent/controller.go	0.00%	1 Missing ⚠️
internal/controller/datadogagent_controller.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1514      +/-   ##
==========================================
+ Coverage   48.63%   48.64%   +0.01%     
==========================================
  Files         227      227              
  Lines       20223    20231       +8     
==========================================
+ Hits         9836     9842       +6     
- Misses       9871     9872       +1     
- Partials      516      517       +1     

Flag	Coverage Δ
unittests	48.64% <73.91%> (+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/datadog/forwarders_manager.go	0.00% <ø> (ø)
pkg/controller/utils/datadog/metrics_forwarder.go	31.94% <100.00%> (+0.86%)	⬆️
internal/controller/datadogagent/controller.go	53.57% <0.00%> (ø)
internal/controller/datadogagent_controller.go	67.28% <66.66%> (+0.31%)	⬆️
internal/controller/setup.go	51.85% <63.63%> (-0.82%)	⬇️

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[levan-m]

Left one nit comment, otherwise looks good to me!