Alerting: Add first Grafana reserved label grafana_folder (#50262)

* Alerting: Add first Grafana reserved label g_label

g_label holds the title of the folder container the alert. The intention of this label
is to use it as part of the new default notification policy groupBy.

* Add nil check on updateRule labels map

* Disable gocyclo lint on schedule.ruleRoutine

will remove later in a separate refactoring PR to reduce complexity.

* Address doc suggestions

* Update g_folder for rules in folder when folder title changes

* Remove global bus in FolderService

* Modify tests to fit new common g_folder label

* Add changelog entry

* Fix merge conflicts

* Switch GrafanaReservedLabelPrefix from `g_` to `grafana_`

docs/sources/alerting/fundamentals/annotation-label/how-to-use-labels.md

@@ -19,6 +19,16 @@ This topic explains why labels are a fundamental component of alerting.
 - The Alertmanager uses labels to match alerts for [silences]({{< relref "../../silences/" >}}) and [alert groups]({{< relref "../../alert-groups/" >}}) in [notification policies]({{< relref "../../notifications/" >}}).
 - The alerting UI shows labels for every alert instance generated during evaluation of that rule.
 - Contact points can access labels to dynamically generate notifications that contain information specific to the alert that is resulting in a notification.
-- Labels can be added to an [alerting rule]({{< relref "../../alerting-rules/" >}}). These manually configured labels are able to use template functions and reference other labels. Labels added to an alerting rule take precedence in the event of a collision between labels.
+- You can add labels to an [alerting rule]({{< relref "../../alerting-rules/" >}}). Labels are manually configurable, use template functions, and can reference other labels. Labels added to an alerting rule take precedence in the event of a collision between labels (except in the case of [Grafana reserved labels](#grafana-reserved-labels)).
 
 {{< figure src="/static/img/docs/alerting/unified/rule-edit-details-8-0.png" max-width="550px" caption="Alert details" >}}
+
+# Grafana reserved labels
+
+> **Note:** Labels prefixed with `grafana_` are reserved by Grafana for special use. If a manually configured label is added beginning with `grafana_` it may be overwritten in case of collision.
+
+Grafana reserved labels can be used in the same way as manually configured labels. The current list of available reserved labels are:
+
+| Label          | Description                               |
+| -------------- | ----------------------------------------- |
+| grafana_folder | Title of the folder containing the alert. |