This extends the possibilities for manipulating data with the Prometheus
collector without using custom scripts.
Work items:
* Implemented the query_function field in the Prometheus collector.
* Documented the query_function field.
* Updated the tests on the Prometheus collector to include the
query_function field.
Change-Id: If6c2242d876eb40fb9a86a776258d50e7a2a441f
Story: 2006427
Task: 37688
b9ed6085f2