(Doc+) KB Diag (#188167)

👋 howdy, team! 

Expanding our new outline on pulling Support Diagnostics [for
Elasticsearch](https://www.elastic.co/guide/en/elasticsearch/reference/master/diagnostic.html),
this adds a new "Capture Diagnostics" under Kibana > Troubleshooting.

Technically the KB Diag works [down to
v6.5.0](https://github.com/elastic/support-diagnostics/blob/main/src/main/resources/kibana-rest.yml)
but I put 7.11.0 since that's the Task Manager Health's earliest
version.

As needed, this page redirects the user back to [investigating
ES](https://www.elastic.co/guide/en/elasticsearch/reference/master/diagnostic.html)
and [fixing KB
dependencies](https://www.elastic.co/guide/en/kibana/master/access.html#not-ready)
first. And directs them forward to Support's [walkthrough investigating
the output](https://www.elastic.co/blog/troubleshooting-kibana-health).

TIA! 🙏

---------

Co-authored-by: florent-leborgne <florent.leborgne@elastic.co>

docs/user/troubleshooting/diagnostic.asciidoc

@@ -0,0 +1,158 @@
+[[kibana-diagnostic]]
+=== Capturing diagnostics
+++++
+<titleabbrev>Capture diagnostics</titleabbrev>
+++++
+:keywords: Kibana diagnostic, diagnostics
+
+The {kib} https://github.com/elastic/support-diagnostics[Support Diagnostic] 
+tool captures a point-in-time snapshot of {kib} and its Task Manager health. 
+It works on {kib} versions 7.11.0 and above. 
+
+You can use the information captured by the tool to troubleshoot problems with {kib} instances. 
+Check the 
+https://www.elastic.co/blog/troubleshooting-kibana-health[Troubleshooting 
+Kibana Health blog] for examples.
+
+You can generate diagnostic information using this tool before you contact 
+https://support.elastic.co[Elastic Support] or 
+https://discuss.elastic.co[Elastic Discuss] to help get a timely answer. 
+
+[discrete]
+[[kibana-diagnostic-tool-requirements]]
+==== Requirements
+
+-  Java Runtime Environment or Java Development Kit v1.8 or higher.
+
+[discrete]
+[[kibana-diagnostic-tool-access]]
+==== Access the tool
+
+The Support Diagnostic tool is included out-of-the-box as a sub-library in: 
+
+* {ece} - Find the tool under **{ece}** > **Deployment** > **Operations** > 
+**Prepare Bundle** > **{kib}**. 
+* {eck} - Run the tool with https://www.elastic.co/guide/en/cloud-on-k8s/current/k8s-take-eck-dump.html[`eck-diagnostics`].
+
+You can also get the latest version of the tool by downloading the `diagnostics-X.X.X-dist.zip` file from 
+https://github.com/elastic/support-diagnostics/releases/latest[the 
+`support-diagnostic` repo].
+
+
+[discrete]
+[[kibana-diagnostic-capture]]
+==== Capture diagnostic information
+
+To run a {kib} diagnostic: 
+
+. In a terminal, verify that your network and user permissions are sufficient 
+to connect by polling {kib}'s <<task-manager-api-health,Task Manager health>>.
++
+For example, with the parameters `host:localhost`, `port:5601`, and 
+`username:elastic`, you'd use the following curl request. Adapt these parameters to your context.
++
+[source,sh]
+----
+curl -X GET -k -H 'kbn-xsrf: true' -u elastic -p https://localhost:5601/api/task_manager/_health
+----
+// NOTCONSOLE
++
+If you receive an HTTP 200 `OK` response, you can proceed to the 
+next step. If you receive a different response code, you must 
+<<kibana-diagnostic-non-200,diagnose the issue>> before proceeding.
+
+. Using the same environment parameters, run the diagnostic tool script. 
++
+For information about the parameters that you can pass to the tool, refer 
+to the https://github.com/elastic/support-diagnostics#standard-options[diagnostic 
+parameter reference]. 
++
+The following command options are recommended:
++
+**Unix-based systems**
++
+[source,sh]
+----
+sudo ./diagnostics.sh --type kibana-local --host localhost --port 5601 -u elastic -p --bypassDiagVerify --ssl --noVerify
+----
++
+**Windows**
++
+[source,sh]
+----
+sudo .\diagnostics.bat --type kibana-local --host localhost --port 5601 -u elastic -p --bypassDiagVerify --ssl --noVerify
+----
++
+[TIP]
+.Script execution modes
+====
+You can execute the script in three https://github.com/elastic/support-diagnostics#diagnostic-types[modes]: 
+
+* `kibana-local` (default, recommended): Polls the <<api,{kib} API>>, 
+gathers operating system info, and captures cluster and garbage collection (GC) logs. 
+
+* `kibana-remote`: Establishes an SSH session 
+to the applicable target server to pull the same information as `kibana-local`.
+
+* `kibana-api`: Polls the <<api,{kib} API>>. All other data must be 
+collected manually.
+====
+
+. When the script has completed, verify that no errors were logged to 
+`diagnostic.log`. If the log file contains errors, refer to 
+<<kibana-diagnostic-log-errors,Diagnose errors in `diagnostic.log`>>.
+
+. If the script completed without errors, an archive with the 
+format `<diagnostic type>-diagnostics-<DateTimeStamp>.zip` is created in 
+the working directory or the output directory that you have specified. You can 
+review or share the diagnostic archive as needed.
+
+[discrete]
+[[kibana-diagnostic-non-200]]
+==== Diagnose a non-200 response
+
+When you poll your Task Manager health, if you receive any response other 
+than `200 0K`, then the diagnostic tool might not work as intended. Check the possible resolutions based on the error code that you get:
+
+HTTP 401 `UNAUTHENTICATED`::
+Additional information in the error will usually indicate 
+that your `username:password` pair is invalid, or that your {es} `.security` 
+index is unavailable and you need to setup a temporary {es}
+{ref}/file-realm.html[file-based realm] user with `role:superuser` to authenticate.
+
+HTTP 403 `UNAUTHORIZED`::
+Your `username` is recognized but has insufficient permissions to run the 
+diagnostic. Either use a different username or increase the user's privileges.
+
+HTTP 429 `TOO_MANY_REQUESTS` (for example, `circuit_breaking_exception`)::
+The authentication and authorization were successful, but the {es} cluster isn't  
+responding to requests. This issue is usually intermittent and can happen 
+when the cluster is overwhelmed. In this case, resolve {es}'s health first before 
+returning to {kib}. 
+
+HTTP 504 `BAD_GATEWAY`::
+Your network has trouble reaching {kib}. You might be using 
+a proxy or firewall. Consider running the diagnostic tool from a different 
+location, confirming your port, or using an IP instead of a URL domain. 
+
+[discrete]
+[[kibana-diagnostic-log-errors]]
+==== Diagnose errors in `diagnostic.log`
+
+The following are common errors that you might encounter when running 
+the diagnostic tool:
+
+* `Error: Could not find or load main class com.elastic.support.diagnostics.DiagnosticApp`
++
+This indicates that you accidentally downloaded the source code file 
+instead of `diagnostics-X.X.X-dist.zip` from the releases page.
+
+* A `security_exception` that includes `is unauthorized for user`:
++
+The provided user has insufficient admin permissions to run the diagnostic 
+tool. Use another user, or grant the user `role:superuser` privileges.
+
+* `{kib} Server is not Ready yet`
++ 
+This indicates issues with {kib}'s dependencies blocking full start-up. 
+To investigate, check <<not-ready,Troubleshoot {kib} UI error>>. 

docs/user/troubleshooting/index.asciidoc

@@ -5,6 +5,8 @@
 
 * <<kibana-troubleshooting-kibana-server-logs, Using {kib} server logs>>
 * <<kibana-troubleshooting-trace-query, Trace {es} query to the origin in {kib}>>
+* <<kibana-diagnostic, Capture {kib} diagnostic>>
 
 include::using-server-logs.asciidoc[]
 include::trace-query.asciidoc[]
+include::diagnostic.asciidoc[]