doc: add release note for runtime/metrics additions

For #56857.

Change-Id: I03bdba906d271d97ce29874c50d5aba55dc285b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/498075
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

doc/go1.21.html

@@ -697,6 +697,18 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
   </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 -->