add basic test that emits fake data

emit1.sh

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+#!/bin/sh
+exec dune exec --profile=release tests/emit1.exe -- $@

tests/dune

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+
+(executable
+  (name emit1)
+  (libraries unix opentelemetry opentelemetry-client-ocurl))

tests/emit1.ml

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+
+module T = Opentelemetry
+
+let run () =
+  Printf.printf "collector is on %S\n%!" (Opentelemetry_client_ocurl.get_url());
+  let i = ref 0 in
+  while true do
+    Unix.sleepf 2.;
+
+    let gc = Gc.stat() in
+    T.Metrics.(
+      emit [
+        gauge ~name:"i" [int !i];
+        gauge ~name:"major-heap-words" [int gc.Gc.heap_words];
+        sum ~name:"minor-allocated" [float gc.Gc.minor_words];
+      ]);
+
+    incr i;
+
+    (* allocate some stuff *)
+    let _arr = Sys.opaque_identity @@ Array.make (25 * 25551) 42.0 in
+    ignore _arr;
+  done
+
+let () =
+  Opentelemetry_client_ocurl.with_setup run