add metric for max goroutines set in the operator

[mrdoggopat]

What does this PR do?

Add a metric for max goroutines set in the operator.

Motivation

What inspired you to submit this pull request?
CECO-1181 - sending a metric for max goroutines be helpful as we could graph that in a dashboard so it’s easier to see when goroutines > max goroutines

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?
N/A

Describe your test plan

Can adjust the maximumGoroutines configuration by adding - --maximumGoroutines=<desired max goroutines> in the args: https://github.com/DataDog/datadog-operator/blob/main/config/manager/manager.yaml#L38

Then deploy the DatadogAgent and exec into the agent container and run curl http://<datadog operator pod IP>:8080/metrics | grep max_goroutines to see if it reflects the same.

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

[alai97]

Looks good for docs

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 48.92%. Comparing base (cfaff9d) to head (df678ee).

Files with missing lines	Patch %	Lines
cmd/main.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1427   +/-   ##
=======================================
  Coverage   48.92%   48.92%           
=======================================
  Files         222      223    +1     
  Lines       19432    19438    +6     
=======================================
+ Hits         9508     9511    +3     
- Misses       9435     9438    +3     
  Partials      489      489           

Flag	Coverage Δ
unittests	48.92% <50.00%> (+<0.01%)	⬆️

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

Files with missing lines	Coverage Δ
internal/controller/metrics/datadogoperator.go	100.00% <100.00%> (ø)
cmd/main.go	0.00% <0.00%> (ø)

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