add clock

src/core/clock.ml

@@ -0,0 +1,26 @@
+open Opentelemetry_atomic
+
+type t = { now: unit -> Timestamp_ns.t } [@@unboxed]
+(** A clock: can get the current timestamp, with nanoseconds precision *)
+
+let[@inline] now (self : t) : Timestamp_ns.t = self.now ()
+
+(** Clock using {!Unix.gettimeofday} *)
+let unix : t =
+  { now = (fun () -> Int64.of_float (Unix.gettimeofday () *. 1e9)) }
+
+module Main = struct
+  open struct
+    let main : t Atomic.t = Atomic.make unix
+  end
+
+  let[@inline] get () = Atomic.get main
+
+  let set t : unit = Util_atomic.update_cas main (fun _ -> (), t)
+
+  (** Clock that always defers to the current main clock *)
+  let dynamic_main : t = { now = (fun () -> now (get ())) }
+end
+
+(** Timestamp using the main clock *)
+let[@inline] now_main () = now (Main.get ())