Setup mtail (#388)

Co-authored-by: holger krekel <holger@merlinux.eu>

CHANGELOG.md

@@ -2,6 +2,16 @@
 
 ## untagged
 
+- add mtail support (new optional `mail_address` ini value)
+  This defines the address on which [`mtail`](https://google.github.io/mtail/)
+  exposes its metrics collected from the logs.
+  If you want to collect the metrics with Prometheus,
+  setup a private network (e.g. WireGuard interface)
+  and assign an IP address from this network to the host.
+  If you do not plan to collect metrics,
+  keep this setting unset.
+  ([#388](https://github.com/deltachat/chatmail/pull/388))
+
 - fix checking for required DNS records
   ([#412](https://github.com/deltachat/chatmail/pull/412))