0.4

• expose Scope.get_surrounding

• rehault of GC metrics

• Trace.with_ now has force_new_trace_id param

• use thread-local storage to store global scope

• add Span_link module

• add a Globals.default_span_kind ref to specify default span kind

• fix(otel-lwt): missing modules now re-exported

• fix(client-ocurl): fix tick version used in the absence of bg thread

• drop dep on ocaml-protoc as the generated code is checked-in

• update vendored otel to 0.19