prometheus-mixin: Make PrometheusRemoteWriteBehind more generic
Currently, it relies on `job, instance` being the labels completely
identifying a Prometheus instance. However, what's intended is to
simply not match on `remote_name, url`.
# https://www.robustperception.io/alerting-on-gauges-in-prometheus-2-0 for details.
(
max_over_time(prometheus_remote_storage_highest_timestamp_in_seconds{%(prometheusSelector)s}[5m])
- - on(job, instance) group_right
+ - ignoring(remote_name, url) group_right
max_over_time(prometheus_remote_storage_queue_highest_sent_timestamp_seconds{%(prometheusSelector)s}[5m])
)
> 120
