Add client metrics documentation

Add docs page for the new client metrics push feature covering what
is collected, how to enable via dashboard, and env var override.

src/components/NavigationDocs.jsx

@@ -438,6 +438,10 @@ export const docsNavigation = [
           },
           { title: 'Auto Update', href: '/manage/peers/auto-update' },
           { title: 'Lazy Connections', href: '/manage/peers/lazy-connection' },
+          {
+            title: 'Client Metrics',
+            href: '/manage/client-metrics',
+          },
           {
             title: 'Notifications',
             href: '/manage/settings/notifications',