The use of a non-threadsafe mutable reference for the metrics callbacks
was resulting in a race condition that would sometimes produce
non-deterministic results in the integration tests.
This has not affected the lwt-based collector, because of the single
threaded concurrency Lwt enforces, but it began to show up in the WIP
Eio rewrite, for which I am testing on cross-domain programs.
I suspect this may have also bee affecting the ocurl collector, but we
don't have integration test running on that yet.
As discussed in https://github.com/ocaml/ocaml/issues/14078,
alerts (with deprecation alerts as a special case) are not currently
supported as item-attributes on let-bindings. This usage produces
`misplaced-attribute` warnings, such as
```
Warning 53 [misplaced-attribute]: the "deprecated" attribute cannot appear in this context
File "src/core/opentelemetry.ml", line 1229, characters 37-47:
1229 | let add_event = Scope.add_event [@@deprecated "use Scope.add_event"]
^^^^^^^^^^
Warning 53 [misplaced-attribute]: the "deprecated" attribute cannot appear in this context
File "src/core/opentelemetry.ml", line 1231, characters 37-47:
1231 | let add_attrs = Scope.add_attrs [@@deprecated "use Scope.add_attrs"]
^^^^^^^^^^
Warning 53 [misplaced-attribute]: the "deprecated" attribute cannot appear in this context
```
Fortunately, we can still add deprecation alerts to these value by
moving the alert to the pattern, as done here.
it's cleaner to emit GC events on ticks rather than on GC,
because it avoids both spamming if the GC is very active, and
emitting nothing when there are few allocations.
