Provide mechanism to specify how often each metric should be collected

[markxnelson]

While you would normally want to collect all metrics at each scrape, it is possible, and likely that there may be some metrics you may wish to collect less frequently, since the collection of the data for those metrics is much more expensive.

This enhancement requests seeks a mechanism to add an interval (or perhaps a set of times) to determine when a metric should be collected. If this is not specified in the definition of a metric, then the default interval should apply.