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

Author: Folyd

tracing-subscriber/src/fmt/fmt_subscriber.rs

@@ -11,7 +11,7 @@ use std::{
 };
 use tracing_core::{
     field,
-    span::{Attributes, Id, Record},
+    span::{Attributes, Current, Id, Record},
     Collect, Event, Metadata,
 };
 
@@ -808,6 +808,11 @@ 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`.
     ///