Trace_debug.Track_spansHelper to track which spans never get closed.
val track :
?on_lingering_spans:[ `Out of out_channel | `Call of unclosed_spans -> unit ] ->
Trace_core.Collector.t ->
Trace_core.Collector.tModify the enter/exit span functions to track the set of spans that are open, and warn at the end if some are not closed.
implementation notes: for now this uses a regular Hashtbl protected by a mutex, so runtime overhead isn't trivial.