Add APM data stream rerouting docs

Closes elastic/apm-server#12375

docs/en/observability/apm/data-streams.asciidoc

@@ -85,6 +85,31 @@ Logs are stored in the following data streams:
 - APM app logging: `logs-apm.app.<service.name>-<namespace>`
 // end::logs-data-streams[]
 
+[discrete]
+[[apm-data-stream-rerouting]]
+=== APM data stream rerouting
+
+APM supports rerouting APM data to user-defined APM data stream names other than the defaults described above.
+This can be achieved by using a `reroute` processor in ingest pipelines to set the data stream dataset or namespace.
+
+For example, consider traces that would originally be indexed to `traces-apm-default`. To set the data stream namespace from the trace's `service.environment` and fallback to a static string `"default"` if `service.environment` does not exist, add an ingest pipeline named `traces-apm@custom`:
+
+[source, json]
+----
+[
+  {
+    "reroute": {
+      "namespace": [
+        "{{service.environment}}",
+        "default"
+      ]
+    }
+  }
+]
+----
+
+For more about reroute processor, see {es-ref}/reroute-processor.html[Reroute processor reference].
+
 [discrete]
 [[apm-data-streams-next]]
 === What's next?