[AO] Fix custom equations to support repeating the same variable (#166482)

## Summary

This PR fixes #166476 by fixing a bug where you can't repeat a variable
twice in the equation. This PR changes the code that converts the
variables from `A` to `params.A` by changing a `replace` to a
`replaceAll`.

The work around would be to duplicate the aggregation that needs to be
repeated so it has a unique variable name.

### Checklist

- [X] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

---------

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

x-pack/plugins/infra/server/lib/create_custom_metrics_aggregations.ts

@@ -74,6 +74,6 @@ export const createCustomMetricsAggregations = (
 const convertEquationToPainless = (bucketsPath: { [id: string]: string }, equation?: string) => {
   const workingEquation = equation || Object.keys(bucketsPath).join(' + ');
   return Object.keys(bucketsPath).reduce((acc, key) => {
-    return acc.replace(key, `params.${key}`);
+    return acc.replaceAll(key, `params.${key}`);
   }, workingEquation);
 };

x-pack/plugins/observability/server/lib/rules/custom_threshold/lib/create_custom_metrics_aggregations.ts

@@ -74,6 +74,6 @@ export const createCustomMetricsAggregations = (
 const convertEquationToPainless = (bucketsPath: { [id: string]: string }, equation?: string) => {
   const workingEquation = equation || Object.keys(bucketsPath).join(' + ');
   return Object.keys(bucketsPath).reduce((acc, key) => {
-    return acc.replace(key, `params.${key}`);
+    return acc.replaceAll(key, `params.${key}`);
   }, workingEquation);
 };

x-pack/test/alerting_api_integration/observability/custom_threshold_rule/custom_eq_avg_bytes_fired.ts

@@ -94,7 +94,7 @@ export default function ({ getService }: FtrProviderContext) {
                   { name: 'A', field: 'system.network.in.bytes', aggType: Aggregators.AVERAGE },
                   { name: 'B', field: 'system.network.out.bytes', aggType: Aggregators.AVERAGE },
                 ],
-                equation: 'A / B ',
+                equation: '(A + A) / (B + B)',
               },
             ],
             alertOnNoData: true,
@@ -187,7 +187,7 @@ export default function ({ getService }: FtrProviderContext) {
                   { name: 'A', field: 'system.network.in.bytes', aggType: Aggregators.AVERAGE },
                   { name: 'B', field: 'system.network.out.bytes', aggType: Aggregators.AVERAGE },
                 ],
-                equation: 'A / B ',
+                equation: '(A + A) / (B + B)',
               },
             ],
             alertOnNoData: true,

x-pack/test/api_integration/apis/metrics_ui/metric_threshold_alert.ts

@@ -153,7 +153,7 @@ export default function ({ getService }: FtrProviderContext) {
                   { name: 'A', aggType: 'count', filter: 'event.dataset: "apache2.error"' },
                   { name: 'B', aggType: 'count' },
                 ],
-                equation: '(A / B) * 100',
+                equation: '((A + A) / (B + B)) * 100',
                 label: 'apache2 error ratio',
               } as CustomMetricExpressionParams,
             ],
@@ -187,7 +187,7 @@ export default function ({ getService }: FtrProviderContext) {
                   { name: 'A', aggType: 'count', filter: 'event.dataset: "apache2.error"' },
                   { name: 'B', aggType: 'count' },
                 ],
-                equation: '(A / B) * 100',
+                equation: '((A + A) / (B + B)) * 100',
                 currentValue: 36.195262024407754,
                 timestamp: '2021-10-19T00:53:59.997Z',
                 shouldFire: true,

x-pack/test/api_integration/apis/metrics_ui/metrics_explorer.ts

@@ -134,7 +134,7 @@ export default function ({ getService }: FtrProviderContext) {
                 { name: 'A', aggregation: 'avg', field: 'system.cpu.user.pct' },
                 { name: 'B', aggregation: 'avg', field: 'system.cpu.user.pct' },
               ],
-              equation: '(A + B) * 100',
+              equation: '((A + A + B + B) / 2) * 100',
             },
           ],
         };

x-pack/test_serverless/api_integration/test_suites/observability/custom_threshold_rule/custom_eq_avg_bytes_fired.ts

@@ -95,7 +95,7 @@ export default function ({ getService }: FtrProviderContext) {
                   { name: 'A', field: 'system.network.in.bytes', aggType: Aggregators.AVERAGE },
                   { name: 'B', field: 'system.network.out.bytes', aggType: Aggregators.AVERAGE },
                 ],
-                equation: 'A / B ',
+                equation: '((A + A) / (B + B))',
               },
             ],
             alertOnNoData: true,
@@ -186,7 +186,7 @@ export default function ({ getService }: FtrProviderContext) {
                   { name: 'A', field: 'system.network.in.bytes', aggType: Aggregators.AVERAGE },
                   { name: 'B', field: 'system.network.out.bytes', aggType: Aggregators.AVERAGE },
                 ],
-                equation: 'A / B ',
+                equation: '((A + A) / (B + B))',
               },
             ],
             alertOnNoData: true,