Round the result of avg_over_time (#152)

This is so that the severity label is always set. Else PagerDuty's API
fails to send the notification when the average is transitioning between
integers, and it causes the AlertmanagerNotificationsFailed alert to
fire.

lib/charms/nrpe_exporter/v0/nrpe_exporter.py

@@ -475,7 +475,9 @@ def _generate_alert(self, relation, cmd, id, unit, nagios_host_context) -> dict:
         return {
             "alert": "{}NrpeAlert".format("".join([x.title() for x in cmd.split("_")])),
             # Average over 5 minutes considering a 60-second scrape interval
-            "expr": f"avg_over_time(command_status{{juju_unit='{unit_label}',command='{cmd}'}}[15m]) > 1"
+            # We need to "round" so the severity label is always set. This is
+            # necessary for PagerDuty's dynamic notifications.
+            "expr": f"round(avg_over_time(command_status{{juju_unit='{unit_label}',command='{cmd}'}}[15m])) > 1"
             + f" or (absent_over_time(command_status{{juju_unit='{unit_label}',command='{cmd}'}}[10m]) == 1)"
             + f" or (absent_over_time(up{{juju_unit='{unit_label}'}}[10m]) == 1)",
             "for": "0m",