apm: Document apm-server.sampling.tail.ttl (#4914)

Author: carsonip

docs/en/observability/apm/configure/sampling.asciidoc

@@ -76,6 +76,18 @@ Default: `3GB`. (text)
 | Fleet-managed     | `Storage limit`
 |====
 
+[float]
+[id="sampling-tail-ttl-{input-type}"]
+=== TTL
+Time-to-live (TTL) for trace events stored in the local storage of the APM Server during tail-based sampling. This TTL determines how long trace events are retained in the local storage while waiting for a sampling decision to be made. A greater TTL value increases storage space requirements. Should be at least 2 * Interval (`sampling.tail.interval`).
+
+Default: `30m` (30 minutes). (duration)
+
+|====
+| APM Server binary | `sampling.tail.ttl`
+| Fleet-managed     | `TTL`
+|====
+
 // end::tbs-top[]
 
 [float]