Disable bfetch in serverless (#183096)

## Summary

Part of #181938.

Disables bfetch in serverless by overriding the `bfetch:disable`
advanced setting in the serverless.yml.

Bfetch was introduced to bypass the browser connection limit when
multiple search requests are made. Here's a picture of what the network
tab looks like with bfetch turned off in http1:


![image](https://github.com/elastic/kibana/assets/1178348/40e156d1-1dc8-4a03-9d1d-cc6c1d61134e)

As the number of requests reaches the connection limit (6 on most modern
browsers), requests stall until a connection is available.

In Cloud/serverless, we have a http2 proxy. Http2 does not have this
browser connection limit. Turning off bfetch allows us to collect
metrics to decide if we want to remove it altogether:

| Bfetch enabled | Bfetch disabled (this PR) |
| --------------- | ------------------------- |
|
![image](https://github.com/elastic/kibana/assets/1178348/09c0673a-573f-422a-b95d-4fa1adcc08b5)
|
![image](https://github.com/elastic/kibana/assets/1178348/0a44f8a4-d047-4b93-afc4-3fdb8a83b494)
|

In my tests, the overall time for a sample dashboard to load was
actually smaller with bfetch disabled on http2 vs. bfetch enabled.

### Checklist

- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [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

config/serverless.yml

@@ -141,6 +141,7 @@ uiSettings:
   overrides:
     # Disables ESQL in advanced settings (hides it from the UI)
     enableESQL: true
+    bfetch:disable: true
     # Disables `Defer loading panels below "the fold"`
     labs:dashboard:deferBelowFold: false
 

x-pack/test_serverless/functional/test_suites/common/discover/group3/_request_counts.ts

@@ -36,7 +36,7 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
       );
       await kibanaServer.uiSettings.replace({
         defaultIndex: 'logstash-*',
-        'bfetch:disable': true,
+        // 'bfetch:disable': true, // bfetch is already disabled in serverless
         // TODO: Removed ES|QL setting since ES|QL isn't supported in Serverless
       });
       await PageObjects.timePicker.setDefaultAbsoluteRangeViaUiSettings();

x-pack/test_serverless/functional/test_suites/common/examples/search_examples/search_example.ts

@@ -20,20 +20,22 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
       await PageObjects.svlCommonPage.loginAsAdmin();
     });
 
-    describe('with bfetch', () => {
-      testSearchExample();
-    });
+    // bfetch is disabled in serverless
+    // describe('with bfetch', () => {
+    //   testSearchExample();
+    // });
 
     describe('no bfetch', () => {
-      const kibanaServer = getService('kibanaServer');
-      before(async () => {
-        await kibanaServer.uiSettings.replace({
-          'bfetch:disable': true,
-        });
-      });
-      after(async () => {
-        await kibanaServer.uiSettings.unset('bfetch:disable');
-      });
+      // No need to disable since it is disabled in serverless.yml
+      // const kibanaServer = getService('kibanaServer');
+      // before(async () => {
+      //   await kibanaServer.uiSettings.replace({
+      //     'bfetch:disable': true,
+      //   });
+      // });
+      // after(async () => {
+      //   await kibanaServer.uiSettings.unset('bfetch:disable');
+      // });
 
       testSearchExample();
     });