fix: wait for cleanup in cohttp client

[c-cube]

in Opentelemetry_client_cohttp_lwt.with_setup we should now wait for
the cleanup to be done, by sneaking in a unit Lwt.u that is only
resolved after the cleanup is done.

close #41

[tatchi]

I tested with the same code as in #41 and cannot reproduce with this change.

I had to slightly adapt the code though due to the breaking API change

diff --git a/experimental/coco/main.ml b/experimental/coco/main.ml
index 1bebf24487b..58aa21e6386 100644
--- a/experimental/coco/main.ml
+++ b/experimental/coco/main.ml
@@ -9,4 +9,4 @@ let () =
   let config =
     Opentelemetry_client_cohttp_lwt.Config.make ~debug:true ~url_traces:"http://127.0.0.1:4318/v1/traces" ()
   in
-  Opentelemetry_client_cohttp_lwt.with_setup ~config () @@ fun () -> Lwt_main.run (run ())
+  Opentelemetry_client_cohttp_lwt.with_setup ~config () run |> Lwt_main.run

You'll have to adapt it as well in the tests:

diff --git a/tests/bin/cohttp_client.ml b/tests/bin/cohttp_client.ml
index dcd395e..8cd4dbb 100644
--- a/tests/bin/cohttp_client.ml
+++ b/tests/bin/cohttp_client.ml
@@ -70,5 +70,4 @@ let () =
     "Check HTTP requests at \
      https://requestbin.com/r/enec1hql02hz/26qShWryt5vJc1JfrOwalhr5vQt@.";
 
-  Opentelemetry_client_cohttp_lwt.with_setup ~config () (fun () ->
-      Lwt_main.run (run ()))
+  Opentelemetry_client_cohttp_lwt.with_setup ~config () run |> Lwt_main.run
diff --git a/tests/bin/emit1_cohttp.ml b/tests/bin/emit1_cohttp.ml
index deb5359..849ff37 100644
--- a/tests/bin/emit1_cohttp.ml
+++ b/tests/bin/emit1_cohttp.ml
@@ -139,5 +139,5 @@ let () =
         Printf.printf "\ndone. %d spans in %.4fs (%.4f/s)\n%!"
           (Atomic.get num_tr) elapsed n_per_sec)
   in
-  Opentelemetry_client_cohttp_lwt.with_setup ~stop ~config () @@ fun () ->
-  Lwt_main.run @@ run ()
+  Opentelemetry_client_cohttp_lwt.with_setup ~stop ~config () run
+  |> Lwt_main.run

[c-cube]

Fixed the tests. So, would you consider this to be an adequate fix for #41?