[prometheus] Add prometheus datetime metrics

src/content/docs/components/prometheus.mdx

@@ -49,6 +49,9 @@ prometheus:
 - Cover
 - Switch
 - Lock
+- Datetime
+- Date
+- Time
 - Text Sensor
 - Text
 - Event
@@ -59,6 +62,11 @@ prometheus:
 - Valve
 - Climate
 
+> [!NOTE]
+> The prometheus metrics for datetime based entities (datetime, date, time)
+> are in milliseconds since the Unix epoch for easy use in
+> grafana, [as mentioned in the Grafana Time Units Docs](https://grafana.com/docs/grafana/latest/visualizations/panels-visualizations/configure-standard-options/#time-format-units)
+
 ## Supported Prometheus Labels
 
 The following labels are supported in all Prometheus metrics. Some metrics may have more labels.
@@ -176,6 +184,18 @@ esphome_text_value{id="template_text",area="Dev",node="devidfprometheus",name="T
 #TYPE esphome_event_failed gauge
 esphome_event_failed{id="template_event",area="Dev",node="devidfprometheus",name="Template Event"} 0
 esphome_event_value{id="template_event",area="Dev",node="devidfprometheus",name="Template Event",last_event_type="custom_event_1"} 1.0
+#TYPE esphome_date_value gauge
+#TYPE esphome_date_failed gauge
+esphome_date_failed{id="date",area="Dev",node="devidfprometheus",name="Date"} 0
+esphome_date_value{id="date",area="Dev",node="devidfprometheus",name="Date"} 946800000000
+#TYPE esphome_time_value gauge
+#TYPE esphome_time_failed gauge
+esphome_time_failed{id="time",area="Dev",node="devidfprometheus",name="Time"} 0
+esphome_time_value{id="time",area="Dev",node="devidfprometheus",name="Time"} 2096000
+#TYPE esphome_datetime_value gauge
+#TYPE esphome_datetime_failed gauge
+esphome_datetime_failed{id="datetime",area="Dev",node="devidfprometheus",name="Datetime"} 0
+esphome_datetime_value{id="datetime",area="Dev",node="devidfprometheus",name="Datetime"} 946845296000
 ```
 
 ## See Also