Document metrics migration

doc/migrations/6.2.1_x.x.x.md

@@ -72,3 +72,24 @@ All the remaining hook metrics now have the `hook_` prefix.
 ### Other metric changes
 
 Several metrics have had their names changed. The new metric names have been documented in module documentation pages.
+
+## Upgrade procedure
+
+In order to have metrics available in MongooseIM, perform the following steps.
+Stop the cluster, or individual nodes, if performing a rolling upgrade, and execute the configuration changes, that were described above:
+
+=== "Keep Exometer metrics"
+
+    1. Add an `[instrumentation]` section.
+    2. Replace the `general.all_metrics_are_global` option with [`instrumentation.exometer.all_metrics_are_global`](../configuration/instrumentation.md#instrumentationexometerall_metrics_are_global).
+    3. Configure [Exometer exporters](../configuration/instrumentation.md#exometer-options) in the `instrumentation` section.
+    4. Note that many metrics have new names, and [some have been removed](#hook-metrics).
+
+=== "Move to Prometheus metrics"
+
+    1. Add an `[instrumentation]` section.
+    2. Remove the `general.all_metrics_are_global` option.
+    3. Add Prometheus to the [instrumentation section](../configuration/instrumentation.md).
+    4. Configure a [listener](../listeners/listen-http.md#handler-types-prometheus-mongoose_prometheus_handler) for Prometheus.
+
+Restart the node or cluster.