Improve watchdog alert

This expression fires the Watchdog alert also if the TSDB is up to date and therefore checks the functionality of the full stack. With the former `vector(1)`, only alertmanager needs to be functional to fire. So In case of a full TSDB storage, the Watchdog still fires and the lack of new metrics goes unnoticed.

Author: a-Tell

jsonnet/kube-prometheus/components/mixin/alerts/general.libsonnet

@@ -28,7 +28,7 @@
                 "DeadMansSnitch" integration in PagerDuty.
               |||,
             },
-            expr: 'vector(1)',
+            expr: 'present_over_time(prometheus_tsdb_head_max_time[1m]) != 0',
             labels: {
               severity: 'none',
             },