Which is the best way to monitor an SNMP metric that NPM is not monitoring...

 The best way to monitor an SNMP metric that NPM is not monitoring out-of-the-box is to create a custom poller using the Universal Device Poller (UnDP) tool. A custom poller allows you to monitor specific aspects of your network devices using SNMP, by selecting the OIDs that correspond to the metrics you want to collect. You can then assign the custom poller to the devices you want to monitor, and view the results in the Orion Web Console, as charts, tables, or gauges. You can also use custom pollers to create alerts and reports based on the collected data. References: Monitor custom statistics based on OIDs with Universal Device Pollers in NPM, Create a custom poller, Assign Universal Device Pollers to devices monitored in NPM