Previously url watchers for pod, service and node objects could be mistakenly closed
when service discovery was set up only for endpoints and endpointslice roles,
since watchers for these roles may start start pod, service and node url watchers
with nil apiWatcher passed to groupWatcher.startWatchersForRole().
Now all the url watchers, which belong to a particular groupWatcher, are stopped at once
when this groupWatcher has no apiWatcher subscribers.
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5216
The issue has been introduced in v1.93.5 when addressing https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4850
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