tracing: Add support for counter events

Add the TRACE_COUNTER macro for recording time-varying integer or
floating point counters.

Design doc: https://docs.google.com/document/d/1EWQ7KP6OlxlbU1PwJKniFa3gsYKoGyhI47afZhko2MA/

Change-Id: I86cd61fc4ba947ec3f522aadaa5fdc7c640084e9
diff --git a/CHANGELOG b/CHANGELOG
index 2fc2527..b57847b 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -6,7 +6,8 @@
       by setting the sorting mode to force a full sort.
   UI:
     *
-
+  SDK:
+    * Add support for counter trace points with the TRACE_COUNTER macro.
 
 v13.0 - 2021-03-01:
   Tracing service and probes: