diff --git a/app/prometheus_unix.ml b/app/prometheus_unix.ml
index eeecab9..5bd686b 100644
--- a/app/prometheus_unix.ml
+++ b/app/prometheus_unix.ml
@@ -65,6 +65,8 @@ let listen_prometheus =
 
 let opts = listen_prometheus
 
+let config port = Some port
+
 let () =
   let add (info, collector) =
     CollectorRegistry.(register default) info collector in
diff --git a/app/prometheus_unix.mli b/app/prometheus_unix.mli
index 6aeea48..ca27ae6 100644
--- a/app/prometheus_unix.mli
+++ b/app/prometheus_unix.mli
@@ -25,6 +25,10 @@ val opts : config Cmdliner.Term.t
 (** [opts] is the extra command-line options to offer Prometheus
     monitoring. *)
 
+val config : int -> config
+(** [config port] creates a new configuration for the metrics server.
+    Metrics will be served on port [port]. *)
+
 (** Report metrics for messages logged. *)
 module Logging : sig
   val init :