Align step with interval passed to Prometheus query using rate() (#5839)

public/app/plugins/datasource/prometheus/datasource.ts

@@ -79,7 +79,7 @@ export function PrometheusDatasource(instanceSettings, $q, backendSrv, templateS
       query.expr = templateSrv.replace(target.expr, options.scopedVars, self.interpolateQueryExpr);
       query.requestId = options.panelId + target.refId;
 
-      var interval = target.interval || options.interval;
+      var interval = templateSrv.replace(target.interval, options.scopedVars) || options.interval;
       var intervalFactor = target.intervalFactor || 1;
       target.step = query.step = this.calculateInterval(interval, intervalFactor);
       var range = Math.ceil(end - start);

public/app/plugins/datasource/prometheus/query_ctrl.ts

@@ -68,7 +68,7 @@ class PrometheusQueryCtrl extends QueryCtrl {
       expr: this.templateSrv.replace(this.target.expr, this.panelCtrl.panel.scopedVars, this.datasource.interpolateQueryExpr),
       range_input: rangeDiff + 's',
       end_input: endTime,
-      step_input: '',
+      step_input: this.target.step,
       stacked: this.panelCtrl.panel.stack,
       tab: 0
     };