fix: use arrow function to preserve this and fix template errors #150 (#153)

Author: Loori-R

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## tip
 
+* BUGFIX: fix an issue in the template variable service where accessing the `datasource` property of `undefined` caused a failure. See [this issue](https://github.com/VictoriaMetrics/grafana-datasource/issues/150).
+
 ## [v0.8.0](https://github.com/VictoriaMetrics/grafana-datasource/releases/tag/v0.8.0)
 
 * FEATURE: add variable type selector to optimize API usage, favoring [/api/v1/labels](https://docs.victoriametrics.com/url-examples/#apiv1labels) and [/api/v1/label/.../values](https://docs.victoriametrics.com/url-examples/#apiv1labelvalues) over [/api/v1/series](https://docs.victoriametrics.com/url-examples/#apiv1series). See [this issue](https://github.com/VictoriaMetrics/grafana-datasource/issues/144)

src/variables.ts

@@ -42,7 +42,7 @@ export class PrometheusVariableSupport extends CustomVariableSupport<PrometheusD
 
   editor = VariableQueryEditor;
 
-  query(request: DataQueryRequest<PromVariableQuery>): Observable<DataQueryResponse> {
+  query = (request: DataQueryRequest<PromVariableQuery>): Observable<DataQueryResponse> => {
     // Handling grafana as code from jsonnet variable queries which are strings and not objects
     // Previously, when using StandardVariableSupport
     // the variable query string was changed to be on the expr attribute