Add Feature Flag page with new FQDN setting (#2817)

* Add Feature Flag page with new FQDN setting

* Fix up host.name description

* Touchup

* Fix up the feature flag description

* Add FQDN prereqs

* touchup

* Update docs/en/ingest-management/elastic-agent/configuration/elastic-agent-standalone-features.asciidoc

Co-authored-by: Karen Metts <35154725+karenzone@users.noreply.github.com>

* Revert rephrasing of feature flag intro cd78011

---------

Co-authored-by: Karen Metts <35154725+karenzone@users.noreply.github.com>

docs/en/ingest-management/elastic-agent/configuration/elastic-agent-standalone-features.asciidoc

@@ -0,0 +1,47 @@
+[[elastic-agent-standalone-feature-flags]]
+= Configure feature flags for standalone {agent}s
+
+++++
+<titleabbrev>Feature flags</titleabbrev>
+++++
+
+include::{fleet-repo-dir}/standalone-note.asciidoc[]
+
+The Feature Flags section of the elastic-agent.yml config file contains settings in {agent} that are disabled by default. These may include experimental features, changes to behaviors within {agent} or its components, or settings that could cause a breaking change. For example a setting that changes information included in events might be inconsistent with the naming pattern expected in your configured {agent} output.
+
+To enable any of the settings listed on this page, change the associated `enabled` flag from `false` to `true`.
+
+[source,yaml]
+----
+agent.features:
+  mysetting:
+    enabled: true
+----
+
+[discrete]
+[[elastic-agent-standalone-feature-flag-settings]]
+== Feature flag configuration settings
+
+You can specify the following settings in the Feature Flag section of the
+`elastic-agent.yml` config file.
+
+Fully qualified domain name (FQDN)::
+When enabled, information provided about the current host through the <<host-provider,host.name>> key, in events produced by {agent}, is in FQDN format (`somehost.example.com` rather than `somehost`). This helps you to distinguish between hosts on different domains that have similar names. With `fqdn` enabled, the fully qualified hostname allows each host to be more easily identified when viewed in {kib}.
++
+For FQDN reporting to work as expected, the hostname of the current host must either:
++
+--
+* Have a CNAME entry defined in DNS.
+* Have one of its corresponding IP addresses respond successfully to a reverse DNS lookup.
+--
++
+If neither pre-requisite is satisfied, `host.name` continues to report the hostname of the current host as if the FQDN feature flag were not enabled.
++
+To enable fully qualified domain names set `enabled: true` for the `fqdn` setting:
++
+["source","yaml",subs="attributes"]
+----
+agent.features:
+  fqdn:
+    enabled: true
+----

docs/en/ingest-management/index.asciidoc

@@ -155,6 +155,8 @@ include::elastic-agent/configuration/authentication/ssl-settings.asciidoc[levelo
 
 include::elastic-agent/configuration/elastic-agent-standalone-logging.asciidoc[leveloffset=+2]
 
+include::elastic-agent/configuration/elastic-agent-standalone-features.asciidoc[leveloffset=+2]
+
 include::elastic-agent/configuration/autodiscovery/elastic-agent-kubernetes-autodiscovery.asciidoc[leveloffset=+2]
 
 include::elastic-agent/configuration/autodiscovery/kubernetes-conditions-autodiscover.asciidoc[leveloffset=+3]