Implement a proxy class to manipulate battery time

Because:
- Remaining battery time sensors, as supported by the UPS widget, can
  report remaining times in both minutes and seconds, depending on the
  source MIB.
- The UPS widget should be consistently reporting the remaining time in
  minutes.
- We want logic in the code, not in the template.
- The Django template that drives the widget only accepts sequences of
  Sensor objects, so it seem cleanest to use a proxy object to
  manipulate what the template sees.

python/nav/web/navlets/ups.py

@@ -19,7 +19,7 @@
 from django.db.models import Q
 from django.utils.six.moves.urllib.parse import urlparse
 
-from nav.models.manage import Netbox
+from nav.models.manage import Netbox, Sensor
 from . import Navlet
 
 
@@ -120,3 +120,35 @@ def post(self, request, **kwargs):
         """Save preferences"""
         return super(UpsWidget, self).post(
             request, form=UpsWidgetForm(request.POST))
+
+
+class BatteryTimesProxy:
+    """Proxies access to Sensor objects that represent remaining battery time.
+
+    For consistency, we want the widget to always display remaining time in minutes,
+    but we need to scale the value for sensors that report the remaining time in
+    seconds.
+    """
+    def __init__(self, proxied_sensor: Sensor):
+        self.__proxied = proxied_sensor
+
+    def __getattr__(self, name):
+        return getattr(self.__proxied, name)
+
+    @property
+    def unit_of_measurement(self):
+        """Reports unit as minutes for sensors that measure seconds"""
+        if self.__proxied.unit_of_measurement == Sensor.UNIT_SECONDS:
+            return Sensor.UNIT_MINUTES
+        else:
+            return self.__proxied.unit_of_measurement
+
+    def get_metric_name(self):
+        """Surrounds the metric name in Graphite scale expressions if conversion from
+        seconds to minutes is needed for the proxied sensor.
+        """
+        name = self.__proxied.get_metric_name()
+        if self.__proxied.unit_of_measurement == Sensor.UNIT_SECONDS:
+            return f"round(scale({name},0.0166),0)"
+        else:
+            return name