feat: add levels to Trace_core.

[c-cube]

these levels are used to control the verbosity levels.

[tatchi]

Should we add tests? I'm getting an error when I set the current log level to Info in t1.ml

git diff
diff --git a/test/t1.ml b/test/t1.ml
index 87e4792..a4c6e32 100644
--- a/test/t1.ml
+++ b/test/t1.ml
@@ -1,6 +1,7 @@
 let run () =
   Trace.set_process_name "main";
   Trace.set_thread_name "t1";
+  Trace.set_current_level Trace.Level.Info;
 

File "test/dune", line 3, characters 8-10:
3 |   (name t1)
            ^^
trace-tef error: cannot find span -9223372036854775808
trace-tef error: cannot find span -92

Looks like it's trying to exit a span that wasn't created due to check_level being false

ocaml-trace/src/core/trace_core.ml

Lines 40 to 47 in e81650d

	let[@inline] with_span ?level ?__FUNCTION__ ~__FILE__ ~__LINE__ ?data name f =
	match A.get collector with
	| Some collector when check_level ?level () ->
	with_span_collector_ collector ?__FUNCTION__ ~__FILE__ ~__LINE__ ?data name
	f
	| _ ->
	(* fast path: no collector, no span *)
	f Collector.dummy_span

[tatchi]

Looks like it's trying to exit a span that wasn't created due to check_level being false

I wonder how it will behave with message. Say we have a current log level equals to Info and we do:

Trace.with_span ~level:Trace.Level.Trace "main" @@ fun span ->
  Trace.message ~level:Trace.Level.Info ~span "world";

I'm not sure exactly how message is supposed to work but looking at this code I understand that we want to "attach" a message to a span. But since the span was created with a level > Info, it maybe won't exist?

[c-cube]

I'm not sure exactly how message is supposed to work but looking at this code I understand that we want to "attach" a message to a span. But since the span was created with a level > Info, it maybe won't exist?

Indeed, the span won't exist, so depending on the backend, the message would attach to the closest span with level <= Info. Some backends don't have this notion at all.

[c-cube]

Thanks a lot for finding the bug in t1.ml btw, I just fixed it.