subscriber: Add a public current_span() method for FmtContext (#1290

)

This backports PR #1290 from `master`.

tracing-subscriber/src/fmt/fmt_layer.rs

@@ -8,7 +8,7 @@ use format::{FmtSpan, TimingDisplay};
 use std::{any::TypeId, cell::RefCell, fmt, io, marker::PhantomData, ops::Deref, time::Instant};
 use tracing_core::{
     field,
-    span::{Attributes, Id, Record},
+    span::{Attributes, Current, Id, Record},
     Event, Metadata, Subscriber,
 };
 
@@ -856,6 +856,11 @@ where
         self.ctx.scope()
     }
 
+    /// Returns the current span for this formatter.
+    pub fn current_span(&self) -> Current {
+        self.ctx.current_span()
+    }
+
     /// Returns the [field formatter] configured by the subscriber invoking
     /// `format_event`.
     ///