[apm/known_issues] Add known issues for ILM to DSL switch (#4192)

* Add known issues for ILM to DSL switch

* Apply suggestions from code review

Co-authored-by: Colleen McGinnis <colleen.j.mcginnis@gmail.com>

* Update ki based on recent changes

* update link of fix to the latest one

* Update wording for cause

* Avoid using abbrev for data stream lifecycle

* Apply suggestions from code review

Co-authored-by: Colleen McGinnis <colleen.j.mcginnis@gmail.com>

---------

Co-authored-by: Colleen McGinnis <colleen.j.mcginnis@gmail.com>

docs/en/observability/apm/known-issues.asciidoc

@@ -19,6 +19,82 @@ If applicable, exact error messages linked to this issue so users searching for
 Link to fix
 ////
 
+*Upgrading to v8.15.0 may cause APM indices to lose their lifecycle policy* +
+_Elastic Stack versions: 8.15.0_ +
+_Fixed in Elastic Stack version 8.15.1_
+
+// The conditions in which this issue occurs
+The issue only occurs when _upgrading_ the {stack} to 8.15.0.
+The issue does _not_ occur when creating a _new_ cluster using 8.15.0.
+The issue also does not occur if a custom ILM policy is configured using a custom component template.
+
+// Describe why it happens
+In 8.15.0, APM Server switched to use data stream lifecycle to manage data retention
+for APM indices for new deployments as well as for upgraded deployments with default lifecycle
+configurations. Unfortunately, since any data stream created before 8.15.0 does not have a data
+stream lifecycle configuration, such existing data streams become unmanaged for default
+lifecycle configurations.
+
+// How to fix it
+Upgrading to 8.15.1 should fix any new indices created for the data stream. However,
+indices created in version 8.15.0 would remain unmanaged if the default ILM policy is
+used. One of the following approaches can be adopted to fix the unmanaged indices:
+1. Manually delete the indices when they are no longer needed.
+2. Explicitly configure APM data streams with the default data stream lifecycle config.
+Using this approach would migrate all data streams to use data stream lifecycles,
+which should be equivalent to the default ILM policies:
+
+[source,txt]
+----
+PUT _data_stream/traces-apm-*/_lifecycle
+{
+  "data_retention": "10d"
+}
+
+PUT _data_stream/traces-apm.rum*/_lifecycle
+{
+  "data_retention": "90d"
+}
+
+PUT _data_stream/traces-apm.sampled*/_lifecycle
+{
+  "data_retention": "1h"
+}
+
+PUT _data_stream/metrics-apm.*.1m-*/_lifecycle
+{
+  "data_retention": "90d"
+}
+
+PUT _data_stream/metrics-apm.*.10m-*/_lifecycle
+{
+  "data_retention": "180d"
+}
+
+PUT _data_stream/metrics-apm.*.60m-*/_lifecycle
+{
+  "data_retention": "390d"
+}
+
+PUT _data_stream/metrics-apm.internal-*/_lifecycle
+{
+  "data_retention": "90d"
+}
+
+PUT _data_stream/metrics-apm.app.*/_lifecycle
+{
+  "data_retention": "90d"
+}
+
+PUT _data_stream/logs-apm.*/_lifecycle
+{
+  "data_retention": "10d"
+}
+----
+
+// Link to fix if it exists
+This issue is fixed in 8.15.1 (https://github.com/elastic/elasticsearch/pull/112432[elastic/elasticsearch#112432]).
+
 [[broken-apm-anomaly-rule]]
 *Upgrading to v8.13.0 to v8.13.2 breaks APM anomaly rules* +
 _Elastic Stack versions: 8.13.0, 8.13.1, 8.13.2_ +