[8.x] [RCA] Fix equation for rule condition chart (#193689) (#193888)

# Backport

This will backport the following commits from `main` to `8.x`:
- [[RCA] Fix equation for rule condition chart
(#193689)](#193689)

<!--- Backport version: 9.4.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

<!--BACKPORT [{"author":{"name":"Bena
Kansara","email":"[email protected]"},"sourceCommit":{"committedDate":"2024-09-24T14:59:35Z","message":"[RCA]
Fix equation for rule condition chart (#193689)\n\nResolves
https://github.com/elastic/kibana/issues/193688","sha":"913b6fed50cbbbc6e05811fa013311e98389e7d0","branchLabelMapping":{"^v9.0.0$":"main","^v8.16.0$":"8.x","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["release_note:skip","v9.0.0","ci:project-deploy-observability","Team:obs-ux-management","v8.16.0","backport:version"],"title":"[RCA]
Fix equation for rule condition
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Fix equation for rule condition chart (#193689)\n\nResolves
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Fix equation for rule condition chart (#193689)\n\nResolves
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BACKPORT-->

Co-authored-by: Bena Kansara <[email protected]>

x-pack/plugins/observability_solution/observability/public/utils/investigation_item_helper.ts

@@ -22,12 +22,12 @@ const AggMappingForLens: Record<string, string> = {
 const genLensEqForCustomThresholdRule = (criterion: MetricExpression) => {
   const metricNameResolver: Record<string, string> = {};
 
-  criterion.metrics.forEach(
-    (metric: CustomThresholdExpressionMetric) =>
-      (metricNameResolver[metric.name] = `${
-        AggMappingForLens[metric.aggType] ? AggMappingForLens[metric.aggType] : metric.aggType
-      }(${metric.field ? metric.field : metric.filter ? metric.filter : ''})`)
-  );
+  criterion.metrics.forEach((metric: CustomThresholdExpressionMetric) => {
+    const metricFilter = metric.filter ? `kql='${metric.filter}'` : '';
+    metricNameResolver[metric.name] = `${
+      AggMappingForLens[metric.aggType] ? AggMappingForLens[metric.aggType] : metric.aggType
+    }(${metric.field ? metric.field : metricFilter})`;
+  });
 
   let equation = criterion.equation
     ? criterion.equation