What feature is required to collect the number of comments users posted to a web page?

Similar to custom dimensions, custom metrics can have different scopes. Hit-level custom metrics get associated with all the hit level dimensions with which it was sent. Similarly, product-level custom metrics are associated only with the product with which it was sent. In many cases, you'll have the option to use events, screen views, and/or a custom metric to track your most important metrics. However, custom metrics can produce more flexible and more readable custom reports and as such are a convenient way to track your most important metrics. Tip: Custom dimensions and metrics are only available for properties that have either been enabled for Universal Analytics or contain at least one app reporting view.