subscriber: explain why we always call inner.register_callsite() be…

…fore if statement (tokio-rs#1433)

This backports tokio-rs#1433 from `master`.

* subscriber: explain why we always call `inner.register_callsite()` before if statement

* Apply suggestions from code review

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

tracing-subscriber/src/layer.rs

@@ -736,6 +736,9 @@ where
             return outer;
         }
 
+        // The intention behind calling `inner.register_callsite()` before the if statement
+        // is to ensure that the inner subscriber is informed that the callsite exists
+        // regardless of the outer subscriber's filtering decision.
         let inner = self.inner.register_callsite(metadata);
         if outer.is_sometimes() {
             // if this interest is "sometimes", return "sometimes" to ensure that