[APM] Transaction group agg size config (#26683) (#26848)

* [APM] Fixes #24204 by adding default configs to kibana.yml

* [APM] fixes #25940 by adding APM config to control top transation group agg size

* Revert the default configs added to kibana.yml and define joi validations for `xpack.apm.ui.transactionGroupBucketSize`

* fix broken test for incorrect config

* [APM] add docs entry for `xpack.apm.ui.transactionGroupBucketSize`

docs/settings/apm-settings.asciidoc

@@ -17,6 +17,8 @@ xpack.apm.enabled:: Set to `false` to disabled the APM plugin {kib}. Defaults to
 xpack.apm.ui.enabled:: Set to `false` to hide the APM plugin {kib} from the menu. Defaults to
 `true`.
 
+xpack.apm.ui.transactionGroupBucketSize:: Number of top transaction groups displayed in APM plugin in Kibana. Defaults to `100`.
+
 apm_oss.indexPattern:: Index pattern is used for integrations with Machine Learning and Kuery Bar. It must match all apm indices. Defaults to `apm-*`.
 
 apm_oss.errorIndices:: Matcher for indices containing error documents. Defaults to `apm-*`.

x-pack/plugins/apm/index.js

@@ -51,7 +51,8 @@ export function apm(kibana) {
       return Joi.object({
         // display menu item
         ui: Joi.object({
-          enabled: Joi.boolean().default(true)
+          enabled: Joi.boolean().default(true),
+          transactionGroupBucketSize: Joi.number().default(100)
         }).default(),
 
         // enable plugin

x-pack/plugins/apm/server/lib/transaction_groups/fetcher.test.ts

@@ -17,7 +17,14 @@ describe('transactionGroupsFetcher', () => {
       end: 1528977600000,
       client: clientSpy,
       config: {
-        get: () => 'myIndex' as any
+        get: jest.fn((key: string) => {
+          switch (key) {
+            case 'apm_oss.transactionIndices':
+              return 'myIndex';
+            case 'xpack.apm.ui.transactionGroupBucketSize':
+              return 100;
+          }
+        })
       }
     };
     const bodyQuery = { my: 'bodyQuery' };

x-pack/plugins/apm/server/lib/transaction_groups/fetcher.ts

@@ -53,7 +53,7 @@ export function transactionGroupsFetcher(
           terms: {
             field: `${TRANSACTION_NAME}.keyword`,
             order: { sum: 'desc' },
-            size: 100
+            size: config.get<number>('xpack.apm.ui.transactionGroupBucketSize')
           },
           aggs: {
             sample: {