Loki: Add comment explaining interpolation of some variables at backend (#69460)

* Loki: Add comment explaining interpolation of some variables at backend

* Update content

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

@@ -1148,6 +1148,86 @@ describe('applyTemplateVariables', () => {
     ds.applyTemplateVariables({ expr: '{test}', refId: 'A' }, {});
     expect(spy).toHaveBeenCalledWith('{test}');
   });
+
+  describe('with template and built-in variables', () => {
+    const scopedVars = {
+      __interval: { text: '1m', value: '1m' },
+      __interval_ms: { text: '1000', value: '1000' },
+      __range: { text: '1m', value: '1m' },
+      __range_ms: { text: '1000', value: '1000' },
+      __range_s: { text: '60', value: '60' },
+      testVariable: { text: 'foo', value: 'foo' },
+    };
+
+    it('should not interpolate __interval variables', () => {
+      const templateSrvMock = {
+        getAdhocFilters: jest.fn().mockImplementation((query: string) => query),
+        replace: jest.fn((a: string, ...rest: unknown[]) => a),
+      } as unknown as TemplateSrv;
+
+      const ds = createLokiDatasource(templateSrvMock);
+      ds.addAdHocFilters = jest.fn().mockImplementation((query: string) => query);
+      ds.applyTemplateVariables(
+        { expr: 'rate({job="grafana"}[$__interval]) + rate({job="grafana"}[$__interval_ms])', refId: 'A' },
+        scopedVars
+      );
+      expect(templateSrvMock.replace).toHaveBeenCalledTimes(2);
+      // Interpolated legend
+      expect(templateSrvMock.replace).toHaveBeenCalledWith(
+        undefined,
+        expect.not.objectContaining({
+          __interval: { text: '1m', value: '1m' },
+          __interval_ms: { text: '1000', value: '1000' },
+        })
+      );
+      // Interpolated expr
+      expect(templateSrvMock.replace).toHaveBeenCalledWith(
+        'rate({job="grafana"}[$__interval]) + rate({job="grafana"}[$__interval_ms])',
+        expect.not.objectContaining({
+          __interval: { text: '1m', value: '1m' },
+          __interval_ms: { text: '1000', value: '1000' },
+        }),
+        expect.any(Function)
+      );
+    });
+
+    it('should not interpolate __range variables', () => {
+      const templateSrvMock = {
+        getAdhocFilters: jest.fn().mockImplementation((query: string) => query),
+        replace: jest.fn((a: string, ...rest: unknown[]) => a),
+      } as unknown as TemplateSrv;
+
+      const ds = createLokiDatasource(templateSrvMock);
+      ds.addAdHocFilters = jest.fn().mockImplementation((query: string) => query);
+      ds.applyTemplateVariables(
+        {
+          expr: 'rate({job="grafana"}[$__range]) + rate({job="grafana"}[$__range_ms]) + rate({job="grafana"}[$__range_s])',
+          refId: 'A',
+        },
+        scopedVars
+      );
+      expect(templateSrvMock.replace).toHaveBeenCalledTimes(2);
+      // Interpolated legend
+      expect(templateSrvMock.replace).toHaveBeenCalledWith(
+        undefined,
+        expect.not.objectContaining({
+          __range: { text: '1m', value: '1m' },
+          __range_ms: { text: '1000', value: '1000' },
+          __range_s: { text: '60', value: '60' },
+        })
+      );
+      // Interpolated expr
+      expect(templateSrvMock.replace).toHaveBeenCalledWith(
+        'rate({job="grafana"}[$__range]) + rate({job="grafana"}[$__range_ms]) + rate({job="grafana"}[$__range_s])',
+        expect.not.objectContaining({
+          __range: { text: '1m', value: '1m' },
+          __range_ms: { text: '1000', value: '1000' },
+          __range_s: { text: '60', value: '60' },
+        }),
+        expect.any(Function)
+      );
+    });
+  });
 });
 
 describe('getTimeRange*()', () => {

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

@@ -815,8 +815,9 @@ export class LokiDatasource
 
   // Used when running queries through backend
   applyTemplateVariables(target: LokiQuery, scopedVars: ScopedVars): LokiQuery {
-    // We want to interpolate these variables on backend
-    const { __interval, __interval_ms, ...rest } = scopedVars || {};
+    // We want to interpolate these variables on backend because we support using them in
+    // alerting/ML queries and we want to have consistent interpolation for all queries
+    const { __interval, __interval_ms, __range, __range_s, __range_ms, ...rest } = scopedVars || {};
 
     const exprWithAdHoc = this.addAdHocFilters(target.expr);