transport: unify request modifiers and reduce allocations

Author: victor

tonic/src/transport/channel/service/request_modifiers.rs

@@ -3,6 +3,15 @@ use std::task::{Context, Poll};
 use tower_service::Service;
 use crate::body::Body;
 
+/// A generic request modifier.
+///
+/// `Modifier<M, T>` wraps an inner service `T` and applies the
+/// modifier `M` to each outgoing `Request`.
+///
+/// This type centralizes the boilerplate for implementing
+/// request middleware. A modifier is closure which receives
+/// the request and mutates it before forwarding to the
+/// inner service.
 #[derive(Debug)]
 pub(crate) struct Modifier<M, T> {
     modifier_fn: M,