The current description does not accurately describe when the metric is incremented.
Aside from Alertmanger missing from the configuration, `prometheus_notifications_dropped_total` is incremented when errors occur while sending alert notifications to Alertmanager, or because the notifications queue is full, or because the number of notifications to be sent exceeds the queue capacity.
I think calling these cases 'errors' in a generic sense is more useful than the current description.
This commit extends the notifier to dispatch alert batches
to multiple Alertmanagers concurrently.
It changes the `-alertmanager.url` flag to accept a comma
separated list of URLs and/or to be set multiple times.