Compose web and Prometheus servers via Eio.Fiber.all

Current_web.run and Prometheus_unix.serve now both return fiber bodies.
Run them together with Fiber.all instead of sequencing them.

Author: mtelvers

src/base_images.ml

@@ -101,8 +101,8 @@ let main () config mode slack_path capnp_address auth_config submission_uri stag
     Routes.(s "login" /? nil @--> Current_github.Auth.login auth) ::
     Current_web.routes engine in
   let site = Current_web.Site.v ?authn ~secure_cookies ~has_role ~name:program_name ~refresh_pipeline:60 routes in
-  Prometheus_unix.serve ~sw ~net prometheus_config;
-  Current_web.run ~net ~mode site
+  Eio.Fiber.all
+    (Current_web.run ~net ~mode site :: Prometheus_unix.serve ~net prometheus_config)
 
 (* Command-line parsing *)