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

…fore if statement

tracing-subscriber/src/subscribe.rs

@@ -586,6 +586,9 @@ where
             return outer;
         }
 
+        // The intention behind calling `inner.register_callsite()` before the if statement
+        // was 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
@@ -712,6 +715,9 @@ where
             return outer;
         }
 
+        // The intention behind calling `inner.register_callsite()` before the if statement
+        // was 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