try to work for lwt 5 and lwt 6

src/client/lwt/dune

@@ -25,3 +25,15 @@
   mtime.clock.os
   lwt.unix)
  (synopsis "Lwt-specific helpers for opentelemetry-client"))
+
+(rule
+ (enabled_if
+  (>= %{version:lwt} 6.0))
+ (action
+  (copy types_.ml.6 types_.ml)))
+
+(rule
+ (enabled_if
+  (< %{version:lwt} 6.0))
+ (action
+  (copy types_.ml.5 types_.ml)))

src/client/lwt/notifier_lwt.ml

@@ -5,7 +5,7 @@ module IO = Io_lwt
 type t = {
   notified: bool Atomic.t;
   cond: unit Lwt_condition.t;
-  notification: Lwt_unix.notification;
+  notification: Types_.notification;
   lwt_tid: int;  (** thread ID where lwt runs *)
   deleted: bool Atomic.t;
 }

src/client/lwt/types_.ml.5

@@ -0,0 +1,2 @@
+
+type notification = int

src/client/lwt/types_.ml.6

@@ -0,0 +1,2 @@
+
+type notification = Lwt_unix.notification