feat: add metrics (1/)

[michaelawyu]

What type of PR is this?

/kind test
/kind feature

/kind api-change
/kind flake

What this PR does / why we need it:

This PR is the first part of a series of PRs that add metrics to Fleet networking controllers. As a part of the change, this PR also:

• adds an ExportedSince field to ExportedObjectReference objects (API change)
• refactors integration tests for several controllers, most importantly ServiceExport, EndpointSlice, InternalServiceExport, and EndpointSliceImport
• fixes a number of issues that leads to flaky integration tests

Which issue(s) this PR fixes:

N/A

Requirements:

• run make reviewable for basic local test
• Read and followed fleet-networking's Code of conduct.
• uses conventional commit messages
• [N/A] includes documentation
• [N/A] adds unit tests

How has this code been tested

[x] Unit tests
[x] Integration tests

Special notes for your reviewer

This PR only introduces the API change, makes changes to the tests (specifically refactors the integration tests for ServiceExport + EndpointSlice controllers), and applies some metric collection preparation work. The actual metric collection part will be added in the next PR.

[codecov]

Codecov Report

Merging #128 (ae67727) into main (e5d0041) will increase coverage by 0.59%.
The diff coverage is 88.34%.

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
+ Coverage   78.39%   78.98%   +0.59%     
==========================================
  Files          20       20              
  Lines        2481     2551      +70     
==========================================
+ Hits         1945     2015      +70     
+ Misses        423      421       -2     
- Partials      113      115       +2     

Impacted Files	Coverage Δ
...ntrollers/member/endpointsliceimport/controller.go	76.79% <ø> (+3.31%)	⬆️
...rollers/member/internalserviceexport/controller.go	83.33% <ø> (ø)
pkg/controllers/member/serviceexport/controller.go	77.01% <85.00%> (+1.89%)	⬆️
pkg/controllers/member/endpointslice/controller.go	77.45% <92.50%> (-0.37%)	⬇️
...ntrollers/member/endpointsliceexport/controller.go	83.05% <100.00%> (ø)
pkg/controllers/member/serviceimport/controller.go	69.69% <100.00%> (ø)
...ontrollers/hub/internalserviceimport/controller.go	76.30% <0.00%> (-1.61%)	⬇️
pkg/controllers/multiclusterservice/controller.go	87.07% <0.00%> (+1.02%)	⬆️
... and 3 more

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