Alerting: Add setting to distribute rule group evaluations over time (#80766)

* Simple, per-base-interval jitter

* Add log just for test purposes

* Add strategy approach, allow choosing between group or rule

* Add flag to jitter rules

* Add second toggle for jittering within a group

* Wire up toggles to strategy

* Slightly improve comment ordering

* Add tests for offset generation

* Rename JitterStrategyFrom

* Improve debug log message

* Use grafana SDK labels rather than prometheus labels

docs/sources/setup-grafana/configure-grafana/feature-toggles/index.md

@@ -174,6 +174,7 @@ Experimental features might be changed or removed without prior notice.
 | `kubernetesFeatureToggles`                  | Use the kubernetes API for feature toggle management in the frontend                                                                                                                                                                                                              |
 | `enablePluginsTracingByDefault`             | Enable plugin tracing for all external plugins                                                                                                                                                                                                                                    |
 | `newFolderPicker`                           | Enables the nested folder picker without having nested folders enabled                                                                                                                                                                                                            |
+| `jitterAlertRules`                          | Distributes alert rule evaluations more evenly over time, by rule group                                                                                                                                                                                                           |
 
 ## Development feature toggles