Alerting: Only alert when an exporter is not scrapable for 5 mins or longer

This change reflects my intention of having an alert fire when a node
is not scrapable for five minutes or longer.  I hadn't understood the
default avg() function well enough before, yielding an alert that would
fire too soon (i.e. any time a node was down, even only one minute for a
reboot).

nixos/modules/monitoring/server/grafana-dashboards/meta-monitoring.json

@@ -42,7 +42,7 @@
             },
             "reducer": {
               "params": [],
-              "type": "avg"
+              "type": "count"
             },
             "type": "query"
           }