Add quantity mutation to Prometheus collector

The Prometheus collector was ignoring the mutate definition from
metrics.yml, which may produce wrong resource ratings.

Change-Id: I662f6ac798c20ca43028b3807fa03aacfdcf252b
Signed-off-by: Ning Yao <yaoning@unitedstack.com>
Story: 2008122
Task: 40844

cloudkitty/collector/prometheus.py

@@ -145,6 +145,7 @@ class PrometheusCollector(collector.BaseCollector):
                 self.conf[metric_name]['factor'],
                 self.conf[metric_name]['offset'],
             )
+            qty = ck_utils.mutate(qty, self.conf[metric_name]['mutate'])
 
         return metadata, groupby, qty
 

releasenotes/notes/prometheus-collector-mutate-8da4748b4d1f0b59.yaml

@@ -0,0 +1,4 @@
+---
+issues:
+  - |
+    The Prometheus collector now applies quantity mutation on metrics.