Fix the SidekiqJobsMetricsMissing alert to only alert when there *should* be...

Fix the SidekiqJobsMetricsMissing alert to only alert when there *should* be metrics there.  Fixes false positives on the prometheus-app instances which don't have the mtail job.

c.f. https://www.robustperception.io/absent-alerting-for-scraped-metrics for inspiration.  We have to ignore le, prog, and worker as those labels don't exist in the up metric.

rules/sidekiq-stats-are-not-scrapeds.yml

@@ -10,7 +10,7 @@ groups:
       runbook: troubleshooting/sidekiq_stats_no_longer_showing.md
       title: Sidekiq stats failed to be scraped for the last minute
   - alert: SidekiqJobsMetricsMissing
-    expr: absent(sidekiq_jobs_completion_time_seconds_bucket)
+    expr: up{job="mtail", type="sidekiq"} == 1 unless ignoring(le, prog, worker) sidekiq_jobs_completion_time_seconds_bucket
     for: 2h
     labels:
       severity: s4