add migration section for the new ES client

src/core/MIGRATION_EXAMPLES.md

@@ -27,6 +27,10 @@ APIs to their New Platform equivalents.
     - [Changes in structure compared to legacy](#changes-in-structure-compared-to-legacy)
     - [Remarks](#remarks)
   - [UiSettings](#uisettings)
+  - [Elasticsearch client](#elasticsearch-client)
+    - [Client API Changes](#client-api-changes)
+    - [Accessing the client from a route handler](#accessing-the-client-from-a-route-handler)
+    - [Creating a custom client](#creating-a-custom-client)
 
 ## Configuration
 
@@ -1003,4 +1007,178 @@ setup(core: CoreSetup){
     },
   })
 }
-```
+```
+
+## Elasticsearch client
+
+The new elasticsearch client is a thin wrapper around `@elastic/elasticsearch`'s `Client` class. Even if the API
+is quite close to the legacy client Kibana was previously using, there are some subtle changes to take into account 
+during migration.
+
+[Official documentation](https://www.elastic.co/guide/en/elasticsearch/client/javascript-api/current/index.html) 
+
+### Client API Changes
+
+The most significant changes for the consumers are the following:
+
+- internal / current user client accessors has been renamed and are now properties instead of functions
+  - `callAsInternalUser('ping')` -> `asInternalUser.ping()`
+  - `callAsCurrentUser('ping')` -> `asCurrentUser.ping()`
+
+- the API now reflects the `Client`'s instead of leveraging the string-based endpoint names the `LegacyAPICaller` was using
+
+```ts
+// before
+const body = await client.callAsInternalUser('indices.get', { index: 'id' });
+// after
+const { body } = await client.asInternalUser.indices.get({ index: 'id' });
+```
+
+- calling any ES endpoint now returns the whole response object instead of only the body payload
+
+```ts
+// before
+const body = await legacyClient.callAsInternalUser('get', { id: 'id' });
+// after
+const { body } = await client.asInternalUser.get({ id: 'id' });
+```
+
+Note that more information from the ES response are available:
+
+```ts
+const {
+  body,        // response payload
+  statusCode,  // http status code of the response
+  headers,     // response headers
+  warnings,    // warnings returned from ES
+  meta         // meta information about the request, such as request parameters, number of attempts and so on
+} = await client.asInternalUser.get({ id: 'id' });
+```
+
+- all API methods are now generic to allow specifying the response body type
+
+```ts
+// before
+const body: GetResponse = await legacyClient.callAsInternalUser('get', { id: 'id' });
+```
+
+```ts
+// body is of type `GetResponse`
+const { body } = await client.asInternalUser.get<GetResponse>({ id: 'id' });
+// fallback to `Record<string, any>` if unspecified
+const { body } = await client.asInternalUser.get({ id: 'id' });
+```
+
+- the returned error types changed 
+
+There are no longer specific errors for every HTTP status code (such as `BadRequest` or `NotFound`). A generic
+`ResponseError` with the specific `statusCode` is thrown instead.
+
+```ts
+// before
+import { errors } from 'elasticsearch';
+try {
+  await legacyClient.callAsInternalUser('ping');
+} catch(e) {
+  if(e instanceof errors.NotFound) {
+    // do something
+  }
+}
+``` 
+
+```ts
+// after
+import { errors } from '@elastic/elasticsearch';
+try {
+  await client.asInternalUser.ping();
+} catch(e) {
+  if(e instanceof errors.ResponseError && e.statusCode === 404) {
+    // do something
+  }
+  // also possible, as all errors got a name property with the name of the class,
+  // so this slightly better in term of performances
+  if(e.name === 'ResponseError' && e.statusCode === 404) {
+    // do something
+  }
+}
+```
+
+Please refer to the  [Breaking changes list](https://www.elastic.co/guide/en/elasticsearch/client/javascript-api/current/breaking-changes.html)
+for more information about the changes between the legacy and new client.
+
+### Accessing the client from a route handler
+
+Apart from the API format change, accessing the client from within a route handler
+did not change. As it was done for the legacy client, a preconfigured scoped client 
+bound to the request is accessible using `core` context provider:
+
+```ts
+// before
+router.get(
+  {
+    path: '/my-route',
+  },
+  async (context, req, res) => {
+    const { client } = context.core.elasticsearch.legacy;
+    // call as current user
+    const res = await client.callAsCurrentUser('ping');
+    // call as internal user
+    const res2 = await client.callAsInternalUser('search', options);
+    return res.ok({ body: 'ok' });
+  }
+);
+```
+
+```ts
+// after
+router.get(
+  {
+    path: '/my-route',
+  },
+  async (context, req, res) => {
+    const { client } = context.core.elasticsearch;
+    // call as current user
+    const res = await client.asCurrentUser.ping();
+    // call as internal user
+    const res2 = await client.asInternalUser.search(options);
+    return res.ok({ body: 'ok' });
+  }
+);
+```
+
+### Creating a custom client
+
+The API to create custom clients did not change much:
+
+```ts
+// before
+const customClient = coreStart.elasticsearch.legacy.createClient('my-custom-client', customConfig);
+// do something with the client, such as
+await customClient.callAsInternalUser('ping');
+// custom client are closable
+customClient.close();
+```
+
+```ts
+// after
+const customClient = coreStart.elasticsearch.createClient('my-custom-client', customConfig);
+// do something with the client, such as
+await customClient.asInternalUser.ping();
+// custom client are closable
+customClient.close();
+```
+
+Note that the `plugins` option is now longer available on the new client. As the API is now exhaustive, adding custom
+endpoints using plugins should no longer be necessary. If, for any reasons, one still needs to reach an 
+endpoint not listed on the client API, using `request.transport` is still possible:
+
+```ts
+const { body } = await client.asCurrentUser.transport.request({
+  method: 'get',
+  path: '/my-custom-endpoint',
+  body: { my: 'payload'},
+  querystring: { param: 'foo' }
+})
+```
+
+Remark: the new client creation API is now only available from the `start` contract of the elasticsearch service.