improvements to "Fix Tor DNS EL Freshness description" metrics

The following discussion from !34 (merged) should be addressed:

• @anarcat started a discussion: (+1 comment)

  So I made a working MR on the other side which will impact this if merged (tpo/network-health/metrics/monitoring-and-alerting!19 (merged)). As noted there, I think the alerting rule here would need to be changed to something like:

    - alert: Tor DNS EL Freshness
      expr: (time() - exits_list_timestamp_seconds)/60 > 180
      for: 5m
      labels:
        severity: critical
        team: metrics
      annotations:
        title: Tor DSN EL Freshness
        description: Tor DNS Exits List has not been updated for more than 180 minutes
        summary: Tor DNS Exits List has not been updated in the last 180 minutes or more