fmt: add reload::Handle::with_current (#91)

## Motivation

In some cases, when filter reloading is in use, it can be valuable to
inspect the current value of a filter without reloading it. For example,
this could be used to format the current state, or call filter-specific
methods on it.

## Solution

This branch adds a new `Handle::with_current` method. This method
invokes a closure with a borrowed reference to the filter's current
state and returns the result. This does not require acquiring a write
lock.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tokio-trace-fmt/src/filter/reload.rs

@@ -105,7 +105,17 @@ where
     where
         F: Clone,
     {
-        self.inner.upgrade().map(|inner| inner.read().clone())
+        self.with_current(F::clone).ok()
+    }
+
+    /// Invokes a closure with a borrowed reference to the current filter,
+    /// returning the result (or an error if the filter no longer exists).
+    pub fn with_current<T>(&self, f: impl FnOnce(&F) -> T) -> Result<T, Error> {
+        let inner = self.inner.upgrade().ok_or(Error {
+            kind: ErrorKind::SubscriberGone,
+        })?;
+        let inner = inner.read();
+        Ok(f(&*inner))
     }
 }