Add changelog entry and modify spec

changelogs/unreleased/rc-use_metric_step.yml

+---
+title: Allow defining of metric step in dashboard yml
+merge_request: 28247
+author:
+type: added

spec/frontend/monitoring/store/actions_spec.js

@@ -597,13 +597,13 @@ describe('Monitoring store actions', () => {
 
     describe('with metric defined step', () => {
       beforeEach(() => {
-        metric.step = 10;
+        metric.step = 7;
       });
 
       const expectedParams = {
         start_time: '2019-08-06T12:40:02.184Z',
         end_time: '2019-08-06T20:40:02.184Z',
-        step: 10,
+        step: 7,
       };
 
       it('uses metric step', done => {