[Stack Monitoring] add back monitoring.ui.metricbeat.index and deprecate (#127462)

* add back monitoring.ui.metricbeat.index and deprecate

* add back to test

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

docs/settings/monitoring-settings.asciidoc

@@ -116,6 +116,11 @@ Specifies the name of the indices that are shown on the
 <<logs-monitor-page,*Logs*>> page in *{stack-monitor-app}*. The default value
 is `filebeat-*`.
 
+`monitoring.ui.metricbeat.index`::
+deprecated:[8.1.1]
+Used as a workaround to avoid querying `metricbeat-*` indices which are now no longer queried. The default value
+is `metricbeat-*`.
+
 `monitoring.ui.max_bucket_size`::
 Specifies the number of term buckets to return out of the overall terms list when
 performing terms aggregations to retrieve index and node metrics. For more

x-pack/plugins/monitoring/server/config.test.ts

@@ -100,6 +100,9 @@ describe('config schema', () => {
             "index": "filebeat-*",
           },
           "max_bucket_size": 10000,
+          "metricbeat": Object {
+            "index": "metricbeat-*",
+          },
           "min_interval_seconds": 10,
           "show_license_expiration": true,
         },

x-pack/plugins/monitoring/server/config.ts

@@ -33,6 +33,9 @@ export const configSchema = schema.object({
     logs: schema.object({
       index: schema.string({ defaultValue: 'filebeat-*' }),
     }),
+    metricbeat: schema.object({
+      index: schema.string({ defaultValue: 'metricbeat-*' }),
+    }),
     max_bucket_size: schema.number({ defaultValue: 10000 }),
     elasticsearch: monitoringElasticsearchConfigSchema,
     container: schema.object({

x-pack/plugins/monitoring/server/deprecations.ts

@@ -78,6 +78,7 @@ export const deprecations = ({
     }),
 
     unused('cluster_alerts.allowedSpaces', { level: 'warning' }),
+    unused('monitoring.ui.metricbeat.index', { level: 'warning' }),
 
     // TODO: Add deprecations for "monitoring.ui.elasticsearch.username: elastic" and "monitoring.ui.elasticsearch.username: kibana".
     // TODO: Add deprecations for using "monitoring.ui.elasticsearch.ssl.certificate" without "monitoring.ui.elasticsearch.ssl.key", and