Add upgrade notes

Author: MadLittleMods

docs/upgrade.md

@@ -117,6 +117,25 @@ each upgrade are complete before moving on to the next upgrade, to avoid
 stacking them up. You can monitor the currently running background updates with
 [the Admin API](usage/administration/admin_api/background_updates.html#status).
 
+# Upgrading to v1.136.0
+
+## Metric labels have changed on `synapse_federation_last_received_pdu_time` and `synapse_federation_last_sent_pdu_time`
+
+Previously, the `synapse_federation_last_received_pdu_time` and
+`synapse_federation_last_sent_pdu_time` metrics both used the `server_name` label to
+differentiate between different servers that we send and receive events from.
+
+Since we're now using the `server_name` label to differentiate between different Synapse
+homeserver instances running in the same process, these metrics have been changed as follows:
+
+ - `synapse_federation_last_received_pdu_time` now uses the `origin_server_name` label
+ - `synapse_federation_last_sent_pdu_time` now used the `destination_server_name` label
+
+The Grafana dashboard JSON in `contrib/grafana/synapse.json` has been updated to reflect
+this change but you will need to manually update your own existing Grafana dashboards
+using these metrics.
+
+
 # Upgrading to v1.135.0
 
 ## `on_user_registration` module API callback may now run on any worker