[Custom threshold] Fix the custom equation label on the preview lens …

…chart (#199618)

Fixes #181876

## Summary

This PR fixes the custom equation label on the preview lens chart.

|With label|Without label|With group by field|
|---|---|---|

|![image](https://github.com/user-attachments/assets/b4f1a1e7-8e67-4b24-ae48-379fe393f90a)|![image](https://github.com/user-attachments/assets/3ef2733f-9ce8-4b37-8ecf-c19c1371aced)|![image](https://github.com/user-attachments/assets/d638f49e-98e2-4df7-8852-84dc62b9f739)|

x-pack/plugins/observability_solution/observability/public/components/rule_condition_chart/rule_condition_chart.tsx

@@ -68,6 +68,7 @@ export interface RuleConditionChartExpressions {
   timeSize?: number;
   timeUnit?: TimeUnitChar;
   equation?: string;
+  label?: string;
 }
 export interface RuleConditionChartProps {
   metricExpression: RuleConditionChartExpressions;
@@ -108,6 +109,7 @@ export function RuleConditionChart({
     threshold,
     comparator,
     equation,
+    label,
     warningComparator,
     warningThreshold,
   } = metricExpression;
@@ -332,7 +334,7 @@ export function RuleConditionChart({
     const baseLayer = {
       type: 'formula',
       value: formula,
-      label: formula,
+      label: label ?? formula,
       groupBy,
       format: {
         id: formatId,
@@ -409,6 +411,7 @@ export function RuleConditionChart({
     comparator,
     dataView,
     equation,
+    label,
     searchConfiguration,
     formula,
     formulaAsync.value,