doc: Update changelog and migration guide

As this could be a minor breaking change for some users, it's necessary
to notify them by updating the changelog and migration guide.

CHANGES.md

@@ -54,6 +54,7 @@ Changed:
 - Added `parents` option of `file.mkdir` (#3600, #3601).
 - Added `forced_major_collections` record field to the result of `runtime.gc.stat()` and
   `runtime.gc.quick_stat()` (#3783).
+- Changed the port for the built-in Prometheus exporter to `9599` (#3801).
 
 ---
 

doc/content/migrating.md

@@ -60,6 +60,11 @@ end
 unwanted requests before consuming process time. If you need to see the request's metadata or if the request resolves
 into a valid tile, however, you might need to call `request.resolve` inside your `check_next` script.
 
+### Prometheus
+
+The default port for the Prometheus metrics exporter has changed from `9090` to `9599`.
+As before, you can change it with `settings.prometheus.server.port := <your port value>`.
+
 ## From 2.1.x to 2.2.x
 
 ### References