Metadata is a very important part of MicroProfile Metrics. While it is possible to expose a metric foo with a value of 142, it is not self-describing. An operator seeing that metric can't tell what this is about, what the unit is, and whether 142 is a good value or not.
Metadata is used to provide units and also a description of the metric so that the preceding could now be foo: runtime; 142 seconds. This now allows for correct scaling on the display to two minutes and 22 seconds. And the user receiving an alert relating to this metric can understand that it refers to some runtime timing.