CP/DP Split: Collect telemetry for cp dp split

[bjee19]

Collect telemetry for number of data plane pods, control plane pods, and nginx proxy resources attached to a gateway.

Problem: With the control data plane split, there are a few telemetry metrics which need to be updated or added.

Solution: Update/Add telemetry metrics.

Testing: Manually tested these scenarios:

NginxPodCount:

• Single gateway/nginx pod
• Replica of single gateway
• Separate instances of nginx

ControlPlanePodCount:

• Single NGF/Control plane pod
• Replica of single NGF/Control plane pod

GatewayAttachedNpCount:

• None are attached (does not count the one attached to the gatewayclass)
• Np is attached to a gateway

Closes #3118

Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.69%. Comparing base (6337c97) to head (9cfeda8).
Report is 268 commits behind head on change/control-data-plane-split.

Additional details and impacted files

@@                         Coverage Diff                         @@
##           change/control-data-plane-split    #3352      +/-   ##
===================================================================
- Coverage                            89.74%   86.69%   -3.06%     
===================================================================
  Files                                  109      129      +20     
  Lines                                11150    14855    +3705     
  Branches                                50       62      +12     
===================================================================
+ Hits                                 10007    12879    +2872     
- Misses                                1083     1829     +746     
- Partials                                60      147      +87     

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[bjee19]

Verified changes on AIDF side.