Opentelemetry_core.ClockA clock: can get the current timestamp, with nanoseconds precision
val now : t -> Timestamp_ns.tval ptime_clock : tClock that uses ptime.
val now_ptime : unit -> Timestamp_ns.tSame as now ptime_clock
module Main : sig ... endSingleton clock
val now_main : unit -> Timestamp_ns.tTimestamp using the main clock