fix: service export metrics + corner cases

[michaelawyu]

What type of PR is this?

/kind bug

What this PR does / why we need it:

This PR fixes the following issues:

• Use resource versions rather than generations to track last seen/observed data in service exports, as Kubernetes does not track service generations
• Address a corner racing condition which may lead the controller to unexport services unexpectedly.

It also fixes a minor issue that may lead to flaky endpointSlice controller 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
• adds unit tests

How has this code been tested

[x] unit tests
[x] integration tests

Special notes for your reviewer

[codecov]

Codecov Report

Merging #143 (58bcc6b) into main (1db0d3b) will increase coverage by 0.00%.
The diff coverage is 97.43%.

@@           Coverage Diff           @@
##             main     #143   +/-   ##
=======================================
  Coverage   79.09%   79.09%           
=======================================
  Files          20       20           
  Lines        2712     2727   +15     
=======================================
+ Hits         2145     2157   +12     
- Misses        450      454    +4     
+ Partials      117      116    -1     

Impacted Files	Coverage Δ
pkg/controllers/member/serviceexport/controller.go	78.32% <96.87%> (+1.31%)	⬆️
...rollers/member/internalserviceexport/controller.go	83.84% <100.00%> (ø)
pkg/controllers/hub/serviceimport/controller.go	72.85% <0.00%> (-4.29%)	⬇️
.../controllers/hub/endpointsliceexport/controller.go	69.77% <0.00%> (-2.99%)	⬇️
pkg/controllers/multiclusterservice/controller.go	85.18% <0.00%> (+1.01%)	⬆️
pkg/controllers/member/endpointslice/controller.go	78.41% <0.00%> (+1.43%)	⬆️
...rollers/member/internalmembercluster/controller.go	70.77% <0.00%> (+2.59%)	⬆️

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