Json even formatter in tracing-subscriber does not use locally defined JsonVisitor

[nduchenk]

Hi, thanks in advance.
Is it expected behaviour that json event formatter will produce double metadata for events emitted by log crate ?
In example below it has "target" field and "log.target" and there is no option to remove "log." metadata.

{"timestamp":"2023-03-11T06:23:49.239209Z","level":"INFO","message":"log event","log.target":"playground","log.module_path":"playground","log.file":"src/main.rs","log.line":13,"target":"playground"}

Here is some notes I've found:
In tracing-subscriber Json even formatter

impl<C, N, T> FormatEvent<C, N> for Format<Json, T>

in this part of code where event fields are recorded and serialized:

    if self.format.flatten_event {
        let mut visitor = tracing_serde::SerdeMapVisitor::new(serializer);
        event.record(&mut visitor);
  
        serializer = visitor.take_serializer()?;
    } else {
        use tracing_serde::fields::AsMap;
        serializer.serialize_entry("fields", &event.field_map())?;
    };

Locally defined JsonVisitor is not used. but it has feature part which should remove already recorded metadata from log crate.
But it is under fn record_debug and it seems that those log metadata will go to fn record_str

  #[cfg(feature = "tracing-log")]
  name if name.starts_with("log.") => (),