balance: Add debug logging for p2c decisions (#77)

When debugging load balancer behavior, it's convenient to observe the
individual node selection decisions. To that end, this change requires
that `Load::Metric` implement `fmt::Debug` when used by
`PowerOfTwoChoices`.

tower-balance/src/choose/p2c.rs

@@ -59,14 +59,24 @@ impl PowerOfTwoChoices {
 impl<K, L> Choose<K, L> for PowerOfTwoChoices
 where
     L: Load,
-    L::Metric: PartialOrd,
+    L::Metric: PartialOrd + ::std::fmt::Debug,
 {
     /// Chooses two distinct nodes at random and compares their load.
     ///
     /// Returns the index of the lesser-loaded node.
     fn choose(&mut self, replicas: Replicas<K, L>) -> usize {
         let (a, b) = self.random_pair(replicas.len());
-        if replicas[a].load() <= replicas[b].load() {
+
+        let a_load = replicas[a].load();
+        let b_load = replicas[b].load();
+        trace!(
+            "choose node[{a}]={a_load:?} node[{b}]={b_load:?}",
+            a = a,
+            b = b,
+            a_load = a_load,
+            b_load = b_load
+        );
+        if a_load <= b_load {
             a
         } else {
             b

tower-balance/src/lib.rs

@@ -42,7 +42,7 @@ pub fn power_of_two_choices<D>(loaded: D) -> Balance<D, choose::PowerOfTwoChoice
 where
     D: Discover,
     D::Service: Load,
-    <D::Service as Load>::Metric: PartialOrd,
+    <D::Service as Load>::Metric: PartialOrd + fmt::Debug,
 {
     Balance::new(loaded, choose::PowerOfTwoChoices::default())
 }