fix: use safe prometheus.Register instead of MustRegister in main

[tekenstam]

Summary

• Replaces prometheus.MustRegister(cacheCollector, controllerCollector) in main.go with individual prometheus.Register(...) calls that handle AlreadyRegisteredError gracefully

Why

prometheus.MustRegister panics if the same collector is registered more than once in the same process. While main() is only called once in normal operation, this pattern is fragile:

• If any future test exercises a code path that calls main() or triggers metric registration, it will panic on the second run
• It's inconsistent with best practice for testable Go code

Found during a cross-repo audit after fixing a similar issue in lifecycle-manager (where prometheus.MustRegister in a method called directly from tests was causing EOF errors in Test_Metrics).

Change

// Before
prometheus.MustRegister(cacheCollector, controllerCollector)

// After
if err := prometheus.Register(cacheCollector); err != nil {
    if _, ok := err.(prometheus.AlreadyRegisteredError); !ok {
        setupLog.Error(err, "failed to register cache metrics collector")
        os.Exit(1)
    }
}
if err := prometheus.Register(controllerCollector); err != nil {
    if _, ok := err.(prometheus.AlreadyRegisteredError); !ok {
        setupLog.Error(err, "failed to register controller metrics collector")
        os.Exit(1)
    }
}

No behavior change in production — the first registration always succeeds and the AlreadyRegisteredError branch is never taken. Any other registration error still causes a fatal exit.

Test plan

• go build ./... — compiles cleanly
• go test $(go list ./... | grep -v test-bdd) — all unit tests pass

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.89%. Comparing base (eada4e6) to head (26bcf6a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #522   +/-   ##
=======================================
  Coverage   46.89%   46.89%           
=======================================
  Files          40       40           
  Lines        5879     5879           
=======================================
  Hits         2757     2757           
  Misses       2964     2964           
  Partials      158      158           

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[kevdowney]

The lint check is failing due to it requires go 1.26, we might want to bump that down.