add Trace_lwt optional library

provides a with_span that's lwt friendly, as well as a ambient span
provider.

dune-project

@@ -22,13 +22,14 @@
  (synopsis
   "A lightweight stub for tracing/observability, agnostic in how data is collected")
  (description
-  "ocaml-trace can be used to instrument libraries and programs with low overhead.\n\n              It doesn't do any IO unless a collector is plugged in, which only\n              the final executable should do.")
+  "ocaml-trace can be used to instrument libraries and programs with low overhead. It doesn't do any IO unless a collector is plugged in, which only the final executable should do.")
  (depends
   (ocaml
    (>= 4.08))
   dune)
  (depopts
   unix
+  lwt
   (thread-local-storage (>= 0.2))
   (mtime
    (>= 2.0)))

src/lwt/dune

@@ -0,0 +1,6 @@
+(library
+ (name trace_lwt)
+ (public_name trace.lwt)
+ (optional) ; lwt
+ (libraries trace.core lwt)
+ (synopsis "Interface with lwt"))

src/lwt/trace_lwt.ml

@@ -0,0 +1,30 @@
+(** Optional interface with lwt.
+
+    @since NEXT_RELEASE *)
+
+open Trace_core
+
+let k_ambient_span : span Lwt.key = Lwt.new_key ()
+
+let ambient_span_provider : Trace_core.Ambient_span_provider.t =
+  ASP_some
+    ( (),
+      {
+        get_current_span = (fun () -> Lwt.get k_ambient_span);
+        with_current_span_set_to =
+          (fun () span f ->
+            Lwt.with_value k_ambient_span (Some span) (fun () -> f span));
+      } )
+
+let with_span ?level ?__FUNCTION__ ~__FILE__ ~__LINE__ ?parent ?params ?data
+    name (f : span -> 'a Lwt.t) : 'a Lwt.t =
+  if Trace_core.enabled () then (
+    let span =
+      Trace_core.enter_span ?level ?__FUNCTION__ ~__FILE__ ~__LINE__ ?parent
+        ?params ?data name
+    in
+    let fut = Trace_core.with_current_span_set_to span f in
+    Lwt.on_termination fut (fun () -> Trace_core.exit_span span);
+    fut
+  ) else
+    f Trace_core.Collector.dummy_span