update toc

Author: Han Kang

keps/sig-instrumentation/3498-extending-stability/README.md

@@ -85,6 +85,11 @@ tags, and then generate with `hack/update-toc.sh`.
 - [Proposal](#proposal)
   - [Risks and Mitigations](#risks-and-mitigations)
 - [Design Details](#design-details)
+  - [Semantic of Stability Levels](#semantic-of-stability-levels)
+    - [Internal Metrics](#internal-metrics)
+    - [Alpha Metrics](#alpha-metrics)
+    - [Beta Metrics](#beta-metrics)
+    - [Stable Metrics](#stable-metrics)
   - [Test Plan](#test-plan)
       - [Prerequisite testing updates](#prerequisite-testing-updates)
       - [Unit tests](#unit-tests)