Prometheus: Allow sub-second step in the prometheus datasource (#21861)

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

@@ -990,14 +990,14 @@ describe('PrometheusDatasource', () => {
       expect(res.url).toBe(urlExpected);
     });
 
-    it('step should never go below 1', async () => {
+    it('step should be fractional for sub second intervals', async () => {
       const query = {
         // 6 minute range
         range: { from: time({ minutes: 1 }), to: time({ minutes: 7 }) },
         targets: [{ expr: 'test' }],
         interval: '100ms',
       };
-      const urlExpected = 'proxied/api/v1/query_range?query=test&start=60&end=420&step=1';
+      const urlExpected = 'proxied/api/v1/query_range?query=test&start=60&end=420&step=0.1';
       datasourceRequestMock.mockImplementation(() => Promise.resolve(response));
       ds.query(query as any);
       const res = datasourceRequestMock.mock.calls[0][0];

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

@@ -396,8 +396,13 @@ export class PrometheusDatasource extends DataSourceApi<PromQuery, PromOptions>
   adjustInterval(interval: number, minInterval: number, range: number, intervalFactor: number) {
     // Prometheus will drop queries that might return more than 11000 data points.
     // Calculate a safe interval as an additional minimum to take into account.
-    const safeInterval = Math.ceil(range / 11000);
-    return Math.max(interval * intervalFactor, minInterval, safeInterval, 1);
+    // Fractional safeIntervals are allowed, however serve little purpose if the interval is greater than 1
+    // If this is the case take the ceil of the value.
+    let safeInterval = range / 11000;
+    if (safeInterval > 1) {
+      safeInterval = Math.ceil(safeInterval);
+    }
+    return Math.max(interval * intervalFactor, minInterval, safeInterval);
   }
 
   performTimeSeriesQuery(query: PromQueryRequest, start: number, end: number) {

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

@@ -46,7 +46,7 @@ export class ResultTransformer {
 
     metricLabel = this.createMetricLabel(metricData.metric, options);
 
-    const stepMs = parseInt(options.step, 10) * 1000;
+    const stepMs = parseFloat(options.step) * 1000;
     let baseTimestamp = start * 1000;
 
     if (metricData.values === undefined) {