docs: CHANGELOG.md tracing entry

Added an entry in the change log on adding tracing.

Signed-off-by: Jonathan Woollett-Light <jcawl@amazon.co.uk>

Author: Jonathan Woollett-Light

CHANGELOG.md

@@ -9,6 +9,9 @@
   each individual net device will emit metrics under the label `"net_{iface_id}"`.
   E.g. the associated metrics for the endpoint `"/network-interfaces/eth0"` will
   be available under `"net_eth0"` in the metrics json object.
+- [#4063](https://github.com/firecracker-microvm/firecracker/pull/4063):
+  Adds source-level instrumentation based tracing. See
+  [tracing](./docs/tracing.md) for more details.
 
 ### Changed