adding back pagemap (#22426)

Author: gracepark

lib/warm-server.js

@@ -32,6 +32,7 @@ async function warmServer() {
   const redirects = await dog.loadRedirects(pageList)
 
   statsd.gauge('num_redirects', Object.keys(redirects).length)
+  statsd.gauge('num_page_map', Object.keys(pageMap).length)
   statsd.gauge('num_page_list', Object.keys(pageList).length)
 
   if (process.env.NODE_ENV !== 'test') {