subscriber: implement per-layer filtering

tracing-subscriber/CHANGELOG.md

@@ -1,3 +1,12 @@
+# Unreleased
+
+### Deprecated
+
+- **registry**: `SpanRef::parent_id`, which cannot properly support per-layer
+  filtering. Use `.parent().map(SpanRef::id)` instead. ([#1523])
+
+[#1523]: https://github.com/tokio-rs/tracing/pull/1523
+
 # 0.2.20 (August 17, 2021)
 
 ### Fixed

tracing-subscriber/debug.log

@@ -0,0 +1,2 @@
+Sep 09 10:44:41.182 DEBUG rust_out: this is a message, and part of a system of messages
+Sep 09 10:44:41.182  WARN rust_out: the message is a warning about danger!