message: subscriber: Expose `FmtContext`
  With 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 for noticing this!
diff: @@ -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};
  