Fix race condition in bearertokenauth extension, the dobule removal of watcher

[pavolloffay]

Description

2025-10-02T02:30:59.851Z	error	bearertokenauthextension@v0.135.0/bearertokenauth.go:131	fsnotify: can't remove non-existent watch: /var/run/secrets/kubernetes.io/serviceaccount/token	{"resource": {"service.instance.id": "db1e09f0-e9b5-421e-b9f2-539af8965363", "service.name": "otelcol", "service.version": "0.135.0"}, "otelcol.component.id": "bearertokenauth", "otelcol.component.kind": "extension"}
github.com/open-telemetry/opentelemetry-collector-contrib/extension/bearertokenauthextension.(*bearerTokenAuth).startWatcher
	github.com/open-telemetry/opentelemetry-collector-contrib/extension/bearertokenauthextension@v0.135.0/bearertokenauth.go:131

The error means the code called watcher.Remove(path) on a file that the fsnotify watcher was already no longer watching.
In the specific case of the Kubernetes service account token, this happens because Kubernetes rotates the token by deleting and replacing the file (it's an atomic update).

Link to tracking issue

Fixes

Testing

make otelcontribcol
make docker-otelcontribcol
docker tag localhost/otelcontribcol:latest pavolloffay/otelcolcontrib-bearertokenfix:1 
docker push pavolloffay/otelcolcontrib-bearertokenfix:1

docker run pavolloffay/otelcolcontrib-bearertokenfix:1

Documentation

[atoulme]

Suggested change

	note: Remove error messages `fsnotify: can't remove non-existent watch` when watching kubernetes SA tokens.
	note: "Remove error messages `fsnotify: can't remove non-existent watch` when watching kubernetes SA tokens."

[atoulme]

LGTM