Add HTML meta keywords.

layouts/header.html

@@ -4,7 +4,8 @@
     <meta charset="utf-8">
     <meta http-equiv="X-UA-Compatible" content="IE=edge">
     <meta name="viewport" content="width=device-width, initial-scale=1">
-    <meta name="description" content="Prometheus monitoring system">
+    <meta name="description" content="Prometheus monitoring system and time series database">
+    <meta name="keywords" content="prometheus, monitoring, monitoring system, time series, time series database, alerting, metrics, telemetry">
     <meta name="author" content="Prometheus">
     <link rel="icon" href="../../favicon.ico">