Add changelog

changelogs/9.0.asciidoc

@@ -14,6 +14,7 @@ https://github.com/elastic/apm-server/compare/v\...v9.0.0[View commits]
 
 [float]
 ==== Breaking Changes
+- Change `sampling.tail.storage_limit` default to `0`. While `0` means unlimited local tail-sampling database size, it now enforces a max 90% disk usage on the disk where the data directory is located. Any tail sampling writes after this threshold will be rejected, similar to what happens when tail-sampling database size exceeds a non-0 storage limit. Setting `sampling.tail.storage_limit` to non-0 maintains the existing behavior which limits the tail-sampling database size to `sampling.tail.storage_limit` and does not have the new disk usage threshold check. {pull}15467[15467]
 
 [float]
 ==== Deprecations

changelogs/all-breaking-changes.asciidoc

@@ -7,6 +7,24 @@
 This section describes the breaking changes and deprecations introduced in this release
 and previous minor versions.
 
+// tag::90-bc[]
+[float]
+[[breaking-changes-9.0]]
+=== 9.0
+
+The following breaking changes are introduced in APM version 9.0.0:
+
+- Change `sampling.tail.storage_limit` default to `0`.
+While `0` means unlimited local tail-sampling database size,
+it now enforces a max 90% disk usage on the disk where the data directory is located.
+Any tail sampling writes after this threshold will be rejected,
+similar to what happens when tail-sampling database size exceeds a non-0 storage limit.
+Setting `sampling.tail.storage_limit` to non-0 maintains the existing behavior
+which limits the tail-sampling database size to `sampling.tail.storage_limit`
+and does not have the new disk usage threshold check.
+For more details, see https://github.com/elastic/apm-server/pull/15467[PR #15467]
+// end::90-bc[]
+
 // tag::811-bc[]
 [float]
 [[breaking-changes-8.11]]