add basic benchmark to exercize trace-tef

bench/dune

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+
+(executable
+  (name trace1)
+  (libraries trace.core trace-tef))

bench/trace1.ml

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+module Trace = Trace_core
+
+let ( let@ ) = ( @@ )
+
+let work ~n () : unit =
+  for _i = 1 to n do
+    let@ _sp =
+      Trace.with_span ~__FILE__ ~__LINE__ "outer" ~data:(fun () ->
+          [ "i", `Int _i ])
+    in
+    for _k = 1 to 10 do
+      let@ _sp = Trace.with_span ~__FILE__ ~__LINE__ "inner" in
+      ()
+    done;
+    Thread.delay 1e-6
+  done
+
+let main ~n ~j () : unit =
+  let domains = Array.init j (fun _ -> Domain.spawn (fun () -> work ~n ())) in
+  Array.iter Domain.join domains
+
+let () =
+  let@ () = Trace_tef.with_setup () in
+
+  let n = ref 10_000 in
+  let j = ref 4 in
+
+  let args =
+    [
+      "-n", Arg.Set_int n, " number of iterations";
+      "-j", Arg.Set_int j, " set number of workers";
+    ]
+    |> Arg.align
+  in
+  Arg.parse args ignore "bench1";
+  main ~n:!n ~j:!j ()

bench1.sh

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+#!/bin/sh
+DUNE_OPTS="--profile=release --display=quiet"
+exec dune exec $DUNE_OPTS bench/trace1.exe -- $@