test(registry): Add goleak-based goroutine leak detection

[surinkim]

test(registry): Add goleak-based goroutine leak detection test for Registry.Gather

This commit adds a new test case TestGatherDoesNotLeakGoroutines which verifies that repeated calls to Registry.Gather() do not leak goroutines.

• 100 collectors are registered to a custom registry
• Gather() is called 1000 times
• goleak.VerifyNone ensures that no unexpected goroutines remain

This test serves as a future safeguard to detect goroutine leaks under heavy usage.

[bwplotka]

Generally LGTM -- was there any historical leak that trigger this PR?

One thing to ensure -- does this upset client_golang importers in terms of extra dependency. I assume NO given we only import this in test, importers should not see it (in theory).

[bwplotka]

Thanks!