DynFilterFn: How to filter based on span field value?

[adiba]

Since EnvFilter is broken, I chose to use DynFilterFn to be able to direct events to different log files based on span values.

Unfortunately I cannot find a way to read the field values of a span:

DynFilterFn::new(|metadata, context| {
    // metadata.fields(): only field names, no values.
    // context.current_span(): only field names, no values.
    // context.lookup_current(): seems to contain the data, but how to read a field?
})

[mladedav]

You can't do that, even the looked up context will not contain the data (unless a layer saved it inside an extension).

If you want to filter based on values, you'll need to implement a custom Layer.