subscriber: Expose FmtContext (#425)

With #420, we introduced FmtContext and made it a required parameter on
FormatEvent's primary methods. However, we forgot to export it, making
it impossible to write custom FormatEvent implementations. This PR fixes
that oversight. Thanks to @samscott89 for noticing this!

Signed-off-by: David Barsky [email protected]

tracing-subscriber/src/fmt/mod.rs

@@ -107,7 +107,7 @@ mod fmt_layer;
 pub mod format;
 pub mod time;
 pub mod writer;
-pub use fmt_layer::{FormattedFields, Layer, LayerBuilder};
+pub use fmt_layer::{FmtContext, FormattedFields, Layer, LayerBuilder};
 
 use crate::layer::Layer as _;
 use crate::{filter::LevelFilter, layer, registry::Registry};