[Infra] Remove indexFields from /api/metrics/source endpoint (#189541)

## Summary

This PR removes `indexFields` from `/api/metrics/source endpoint`


![image](https://github.com/user-attachments/assets/496710da-55b8-4ec4-aadc-7bab794a8cfb)

## Testing

Check all infra pages (try creating an alert, open a host as page, etc.)
it should work as before

x-pack/plugins/observability_solution/infra/common/metrics_sources/index.ts

@@ -51,7 +51,6 @@ const metricsSourceConfigurationOriginRT = rt.keyof({
 export const metricsSourceStatusRT = rt.strict({
   metricIndicesExist: SourceStatusRuntimeType.props.metricIndicesExist,
   remoteClustersExist: SourceStatusRuntimeType.props.metricIndicesExist,
-  indexFields: SourceStatusRuntimeType.props.indexFields,
 });
 
 export type MetricsSourceStatus = rt.TypeOf<typeof metricsSourceStatusRT>;

x-pack/plugins/observability_solution/infra/common/source_configuration/source_configuration.ts

@@ -144,7 +144,6 @@ export const SourceStatusRuntimeType = rt.type({
   logIndicesExist: rt.boolean,
   metricIndicesExist: rt.boolean,
   remoteClustersExist: rt.boolean,
-  indexFields: rt.array(SourceStatusFieldRuntimeType),
 });
 
 export interface InfraSourceStatus extends rt.TypeOf<typeof SourceStatusRuntimeType> {}

x-pack/plugins/observability_solution/infra/server/routes/metrics_sources/index.ts

@@ -29,7 +29,6 @@ import { InfraPluginRequestHandlerContext } from '../../types';
 import { getInfraMetricsClient } from '../../lib/helpers/get_infra_metrics_client';
 
 const defaultStatus = {
-  indexFields: [],
   metricIndicesExist: false,
   remoteClustersExist: false,
 };
@@ -51,9 +50,6 @@ export const initMetricsSourceConfigurationRoutes = (libs: InfraBackendLibs) =>
     /**
      * Extract values from promises settlements
      */
-    const indexFields = isFulfilled<InfraSourceIndexField[]>(indexFieldsSettled)
-      ? indexFieldsSettled.value
-      : defaultStatus.indexFields;
     const metricIndicesExist = isFulfilled<boolean>(metricIndicesExistSettled)
       ? metricIndicesExistSettled.value
       : defaultStatus.metricIndicesExist;
@@ -73,7 +69,6 @@ export const initMetricsSourceConfigurationRoutes = (libs: InfraBackendLibs) =>
     }
 
     return {
-      indexFields,
       metricIndicesExist,
       remoteClustersExist,
     };