[Metric threshold rule] Adds new context variable for group by keys (#145654)

## Summary

Closes https://github.com/elastic/kibana/issues/141496

Adds new context variable called `groupByKeys` which holds object of
group by keys for Metric threshold rule.

### Manual Testing

1. Create Metric threshold rule with group by e.g. `host.name`,
`host.network.name`
2. Define action template with `context.groupByKeys`
3. Wait for alerts to be generated
4. Notice the group by object to be included in the alert notification

<img width="883" alt="Screenshot 2022-11-21 at 12 27 20"
src="https://user-images.githubusercontent.com/69037875/203039799-1465a8dc-cdf6-4adb-8fe2-9ced5833c3be.png">

### Checklist
- [x] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

x-pack/plugins/infra/server/lib/alerting/common/messages.ts

@@ -162,6 +162,13 @@ export const groupActionVariableDescription = i18n.translate(
   }
 );
 
+export const groupByKeysActionVariableDescription = i18n.translate(
+  'xpack.infra.metrics.alerting.groupByKeysActionVariableDescription',
+  {
+    defaultMessage: 'The object containing groups that are reporting data',
+  }
+);
+
 export const alertStateActionVariableDescription = i18n.translate(
   'xpack.infra.metrics.alerting.alertStateActionVariableDescription',
   {

x-pack/plugins/infra/server/lib/alerting/common/utils.ts

@@ -294,3 +294,23 @@ export const flattenObject = (
 
     return acc;
   }, {});
+
+export const getGroupByObject = (
+  groupBy: string | string[] | undefined,
+  resultGroupSet: Set<string>
+): Record<string, object> => {
+  const groupByKeysObjectMapping: Record<string, object> = {};
+  if (groupBy) {
+    resultGroupSet.forEach((groupSet) => {
+      const groupSetKeys = groupSet.split(',');
+      groupByKeysObjectMapping[groupSet] = unflattenObject(
+        Array.isArray(groupBy)
+          ? groupBy.reduce((result, group, index) => {
+              return { ...result, [group]: groupSetKeys[index] };
+            }, {})
+          : { [groupBy]: groupSet }
+      );
+    });
+  }
+  return groupByKeysObjectMapping;
+};

x-pack/plugins/infra/server/lib/alerting/metric_threshold/metric_threshold_executor.ts

@@ -36,6 +36,7 @@ import {
   hasAdditionalContext,
   validGroupByForContext,
   flattenAdditionalContext,
+  getGroupByObject,
 } from '../common/utils';
 
 import { EvaluatedRuleParams, evaluateRule } from './lib/evaluate_rule';
@@ -187,6 +188,7 @@ export const createMetricThresholdExecutor = (libs: InfraBackendLibs) =>
       }
     }
 
+    const groupByKeysObjectMapping = getGroupByObject(params.groupBy, resultGroupSet);
     const groups = [...resultGroupSet];
     const nextMissingGroups = new Set<MissingGroupsRecord>();
     const hasGroups = !isEqual(groups, [UNGROUPED_FACTORY_KEY]);
@@ -277,6 +279,7 @@ export const createMetricThresholdExecutor = (libs: InfraBackendLibs) =>
           alertDetailsUrl: getAlertDetailsUrl(libs.basePath, spaceId, alertUuid),
           alertState: stateToAlertMessage[nextState],
           group,
+          groupByKeys: groupByKeysObjectMapping[group],
           metric: mapToConditionsLookup(criteria, (c) => c.metric),
           reason,
           threshold: mapToConditionsLookup(

x-pack/plugins/infra/server/lib/alerting/metric_threshold/register_metric_threshold_rule_type.ts

@@ -18,6 +18,7 @@ import {
   cloudActionVariableDescription,
   containerActionVariableDescription,
   groupActionVariableDescription,
+  groupByKeysActionVariableDescription,
   hostActionVariableDescription,
   labelsActionVariableDescription,
   metricActionVariableDescription,
@@ -106,6 +107,7 @@ export async function registerMetricThresholdRuleType(
     actionVariables: {
       context: [
         { name: 'group', description: groupActionVariableDescription },
+        { name: 'groupByKeys', description: groupByKeysActionVariableDescription },
         ...(getAlertDetailsPageEnabledForApp(config, 'metrics')
           ? [{ name: 'alertDetailsUrl', description: alertDetailUrlActionVariableDescription }]
           : []),