feat: opentelemetry.domain shim

src/domain/dune

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+(library
+ (name opentelemetry_domain)
+ (synopsis "Compatibility package for the Domain module for opentelemetry")
+ (public_name opentelemetry.domain)
+ (modules opentelemetry_domain))
+
+(executable
+ (modules gen)
+ (name gen))
+
+(rule
+ (targets opentelemetry_domain.ml)
+ (action
+  (run ./gen.exe)))

src/domain/gen.ml

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+let domain_4 =
+  {|
+let cpu_relax = ignore
+let relax_loop : int -> unit = ignore
+  |}
+
+let domain_5 =
+  {|
+let cpu_relax = Domain.cpu_relax
+let relax_loop i =
+  for _j = 1 to i do cpu_relax () done
+|}
+
+let write_file file s =
+  let oc = open_out file in
+  output_string oc s;
+  close_out oc
+
+let () =
+  let version = Scanf.sscanf Sys.ocaml_version "%d.%d.%s" (fun x y _ -> x, y) in
+  write_file "opentelemetry_domain.ml"
+    (if version >= (5, 0) then
+       domain_5
+     else
+       domain_4);
+  ()

src/domain/opentelemetry_domain.mli

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+val cpu_relax : unit -> unit
+
+val relax_loop : int -> unit
+(** Call {!cpu_relax} n times *)