builder: Add ServiceBuilder::layer_fn (#560)

This gem as just [sitting in Tonic][sit] waiting to be upstreamed.

[sit]: https://github.com/hyperium/tonic/blob/master/tonic/src/transport/service/layer.rs

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

tower/CHANGELOG.md

@@ -6,6 +6,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 # Unreleased
 
+- **builder**: Add `ServiceBuilder::layer_fn` to add a layer built from a function.
 - **builder**: Add `ServiceBuilder::map_future` for transforming the futures produced
   by a service.
 - **util**: Add example for `service_fn`.

tower/src/builder/mod.rs

@@ -159,6 +159,15 @@ impl<L> ServiceBuilder<L> {
         self.layer(crate::util::option_layer(layer))
     }
 
+    /// Add a [`Layer`] built from a function that accepts a service and returns another service.
+    ///
+    /// See the documentation for [`layer_fn`] for more details.
+    ///
+    /// [`layer_fn`]: crate::layer::layer_fn
+    pub fn layer_fn<F>(self, f: F) -> ServiceBuilder<Stack<crate::layer::LayerFn<F>, L>> {
+        self.layer(crate::layer::layer_fn(f))
+    }
+
     /// Buffer requests when when the next layer is not ready.
     ///
     /// This wraps the inner service with an instance of the [`Buffer`]