Section 3.1.2 of the MP Metrics spec states:

Tags can be supplied in two ways:
...
At the application server level by using MicroProfile Config and setting a configuration property of the name mp.metrics.tags.

This does not work in WildFly if the property is configured inline in the Microprofile Config subsystem like this:

        <subsystem xmlns="urn:wildfly:microprofile-config-smallrye:1.0">
            <config-source name="props">
                <property name="mp.metrics.tags" value="foo=bar"/>
            </config-source>
        </subsystem>

Workaround: specify it under <system-properties> instead. This works as expected:

    <system-properties>
        <property name="mp.metrics.tags" value="foo=bar"/>
    </system-properties>