Several other pages have the ability to add metrics, but the host-level page does not have the "Add Metric" (e.g., the Ambari homepage has the Metric Actions >> Add option, and the service level page for HDFS has the "+" button in the dashboard. Host-level pages seem to lack the option, and the Ambari dashboard seems to only offer a small subset of metrics.
In the widget context, only service page widgets are persisted in ambari db, their definition is REST API driven and So it allows to add user defined widgets. It has "Create Widget" wizard that allows user to do so.
Dashboard page widgets and host page widgets and their definition is still completely coded in ambari-web JS code.
so to add any new widget as of now, new widget definition code needs to be added in ambari-web JS code.
Pointers to the host page widget definition code: https://github.com/apache/ambari/tree/trunk/ambari-web/app/views/main/host/metrics