fix: fixed prometheus step issue that caused browser crash, fixes #9575

(cherry picked from commit 8d68bd6bb9)

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

@@ -168,7 +168,7 @@ export class PrometheusDatasource {
     if (interval !== 0 && range / intervalFactor / interval > 11000) {
       interval = Math.ceil(range / intervalFactor / 11000);
     }
-    return Math.max(interval * intervalFactor, minInterval);
+    return Math.max(interval * intervalFactor, minInterval, 1);
   }
 
   performTimeSeriesQuery(query, start, end) {

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

@@ -294,6 +294,20 @@ describe('PrometheusDatasource', function() {
       ctx.ds.query(query);
       ctx.$httpBackend.verifyNoOutstandingExpectation();
     });
+
+    it('step should never go below 1', function() {
+      var query = {
+        // 6 hour range
+        range: { from: moment(1508318768202), to: moment(1508318770118) },
+        targets: [{expr: 'test'}],
+        interval: '100ms'
+      };
+      var urlExpected = 'proxied/api/v1/query_range?query=test&start=1508318769&end=1508318771&step=1';
+      ctx.$httpBackend.expect('GET', urlExpected).respond(response);
+      ctx.ds.query(query);
+      ctx.$httpBackend.verifyNoOutstandingExpectation();
+    });
+
     it('should be auto interval when greater than min interval', function() {
       var query = {
         // 6 hour range