Hi Yan,
I see your reference to PM-2.5 as your current release level.
I have begun working with PM-2.7 in the last few months, specifically regarding cert-extensions and custom-vendor-certs.
I'm not certain if new MVEL features in 2.7 will be required for your request-issue, but I'd suggest that rather than 'adding a tag metric' i think you likely want to custom-extend the standard Names field to pull in the Tag-String in place of the default-name-code.
The reason for this is because beyond Name and Descr fields, the typical expression fields need to be calculated numeric values.