Add interface for flush spans and exit the export thread

Signed-off-by: Vincent Liu <shuntian.liu2@cloud.com>

Author: Vincent-lau

ocaml/libs/tracing/tracing.ml

@@ -13,6 +13,7 @@
  *)
 module D = Debug.Make (struct let name = "tracing" end)
 
+module Delay = Xapi_stdext_threads.Threadext.Delay
 open D
 
 type endpoint = Bugtool | Url of Uri.t
@@ -803,19 +804,30 @@ module Export = struct
       |> List.concat_map (fun x -> TracerProvider.get_endpoints x)
       |> List.iter (export_to_endpoint parent span_list)
 
+    let delay = Delay.make ()
+
+    (* Note this signal will flush the spans and terminate the exporter thread *)
+    let signal () = Delay.signal delay
+
     let main () =
       enable_span_garbage_collector () ;
       Thread.create
         (fun () ->
           while true do
             debug "Tracing: Waiting %d seconds before exporting spans"
               (int_of_float !export_interval) ;
-            Thread.delay !export_interval ;
+            if not (Delay.wait delay !export_interval) then (
+              debug "Tracing: we are signaled, export spans now and exit" ;
+              flush_spans () ;
+              raise Thread.Exit
+            ) ;
             flush_spans ()
           done
         )
         ()
   end
 end
 
+let flush_and_exit = Export.Destination.signal
+
 let main = Export.Destination.main

ocaml/libs/tracing/tracing.mli

@@ -152,4 +152,6 @@ val set_observe : bool -> unit
 
 val validate_attribute : string * string -> bool
 
+val flush_and_exit : unit -> unit
+
 val main : unit -> Thread.t