Previous commit 9ca74d1fff added a regression for notifier's metrics exposed by vmalert. vmalert returned new notifier instances for the blackhole notifier type. And it registered new metrics each get notifiers function was called. It registered duplicate metrics and lead to OOM crash.
This commit properly init blachole notifier instances and add metrics for it only once, during application start.
Related issue:
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/8532