How to export tokio-metrics with opentelemetry::metrics?

[foo-jin]

I'm trying to export metrics about tokio tasks/runtime from tokio-metrics. It provides an interval iterator to get metrics concerning the time period since the previous interval. My understanding is that these sort of use-cases are what the ObservableXXX Instruments are for. However, this did not work since Meter::register_callback takes an Fn(&dyn Observer) + Send + Sync + 'static, which does not allow mutating the iterator required to call Iterator::next. The only workaround I could think of is to:

1. declare a bunch of static atomics for each of the metrics
2. update the atomics with the values from the interval iterator regularly in a background task
3. read these atomics in the registered callback to push them

This feels a bit unfortunate, especially since the number of metrics is quite high (> 40). Additionally this introduces inaccuracy w.r.t. the timing of the data. Am I going about this the wrong way? And is there a particular reason callbacks can't be FnMut?

[foo-jin]

I've decided to just stick the iterator in a Mutex, since only the metrics collection task should contend on it.