Loki: Timeseries should not produce 0-values for missing data (#30116)

- It should produce NULL values instead, to be in line with Prometheus.

public/app/plugins/datasource/loki/result_transformer.test.ts

 import { CircularDataFrame, FieldCache, FieldType, MutableDataFrame } from '@grafana/data';
 import { LokiStreamResult, LokiTailResponse, LokiStreamResponse, LokiResultType, TransformerOptions } from './types';
 import * as ResultTransformer from './result_transformer';
-import { enhanceDataFrame } from './result_transformer';
+import { enhanceDataFrame, lokiPointsToTimeseriesPoints } from './result_transformer';
 import { setTemplateSrv } from '@grafana/runtime';
 import { TemplateSrv } from 'app/features/templating/template_srv';
 
@@ -256,4 +256,30 @@ describe('enhanceDataFrame', () => {
       url: '',
     });
   });
+
+  describe('lokiPointsToTimeseriesPoints()', () => {
+    /**
+     * NOTE on time parameters:
+     * - Input time series data has timestamps in sec (like Prometheus)
+     * - Output time series has timestamps in ms (as expected for the chart lib)
+     * - Start/end parameters are in ns (as expected for Loki)
+     * - Step is in sec (like in Prometheus)
+     */
+    const data: Array<[number, string]> = [
+      [1, '1'],
+      [2, '0'],
+      [4, '1'],
+    ];
+
+    it('returns data as is if step, start, and end align', () => {
+      const options: Partial<TransformerOptions> = { start: 1 * 1e9, end: 4 * 1e9, step: 1 };
+      const result = lokiPointsToTimeseriesPoints(data, options as TransformerOptions);
+      expect(result).toEqual([
+        [1, 1000],
+        [0, 2000],
+        [null, 3000],
+        [1, 4000],
+      ]);
+    });
+  });
 });

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

@@ -185,7 +185,10 @@ function lokiMatrixToTimeSeries(matrixResult: LokiMatrixResult, options: Transfo
   };
 }
 
-function lokiPointsToTimeseriesPoints(data: Array<[number, string]>, options: TransformerOptions): TimeSeriesValue[][] {
+export function lokiPointsToTimeseriesPoints(
+  data: Array<[number, string]>,
+  options: TransformerOptions
+): TimeSeriesValue[][] {
   const stepMs = options.step * 1000;
   const datapoints: TimeSeriesValue[][] = [];
 
@@ -199,7 +202,7 @@ function lokiPointsToTimeseriesPoints(data: Array<[number, string]>, options: Tr
 
     const timestamp = time * 1000;
     for (let t = baseTimestampMs; t < timestamp; t += stepMs) {
-      datapoints.push([0, t]);
+      datapoints.push([null, t]);
     }
 
     baseTimestampMs = timestamp + stepMs;
@@ -208,7 +211,7 @@ function lokiPointsToTimeseriesPoints(data: Array<[number, string]>, options: Tr
 
   const endTimestamp = options.end / 1e6;
   for (let t = baseTimestampMs; t <= endTimestamp; t += stepMs) {
-    datapoints.push([0, t]);
+    datapoints.push([null, t]);
   }
 
   return datapoints;