diff --git a/doc/go1.21.html b/doc/go1.21.html
index ad84fa04a7..4a771bc6f3 100644
--- a/doc/go1.21.html
+++ b/doc/go1.21.html
@@ -697,6 +697,18 @@ Do not send CLs removing the interior tags from such phrases.
   </dd>
 </dl><!-- runtime/trace -->
 
+<dl id="runtime/metrics"><dt><a href="/pkg/runtime/metrics/">runtime/metrics</a></dt>
+  <dd>
+    <p><!-- https://go.dev/issue/56857 -->
+      A few previously-internal GC metrics, such as live heap size, are
+      now available.
+
+      <code>GOGC</code> and <code>GOMEMLIMIT</code> are also now
+      available as metrics.
+    </p>
+  </dd>
+</dl><!-- runtime/metrics -->
+
 <dl id="slices"><dt><a href="/pkg/slices/">slices</a></dt>
   <dd>
     <p><!-- https://go.dev/issue/54768 -->