wip: tests for prometheus

tests/prometheus/dune

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+
+(test
+  (name t_prom)
+  (libraries tiny_httpd.prometheus))

tests/prometheus/t_prom.expected

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+==== first try ====
+```
+# TYPE yolo_gauge gauge
+yolo_gauge{level="max"} 2525
+# HELP t_c2 more awesome than c1
+# TYPE t_c2 counter
+t_c2 1
+# TYPE t_c1 counter
+t_c1 42
+
+```
+==== second try====
+```
+# TYPE yolo_gauge gauge
+yolo_gauge{level="max"} 42000
+# HELP t_c2 more awesome than c1
+# TYPE t_c2 counter
+t_c2 2
+# TYPE t_c1 counter
+t_c1 53
+
+```

tests/prometheus/t_prom.ml

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+module P = Tiny_httpd_prometheus
+
+let pf = Printf.printf
+let reg = P.Registry.create ()
+let c1 = P.Counter.create reg "t_c1"
+let c2 = P.Counter.create reg "t_c2" ~descr:"more awesome than c1"
+let g1 = P.Gauge.create reg ~tags:[ "level", "max" ] "yolo_gauge"
+
+let () =
+  print_endline "==== first try ====";
+  P.Counter.inc_by c1 42;
+  P.Counter.inc c2;
+  P.Gauge.set g1 2525;
+
+  pf "```\n%s\n```\n" @@ P.Registry.emit_str reg
+
+let () =
+  print_endline "==== second try====";
+  P.Counter.inc_by c1 11;
+  P.Counter.inc c2;
+  P.Gauge.set g1 42_000;
+
+  pf "```\n%s\n```\n" @@ P.Registry.emit_str reg