fix: Resolves issue #46

This ensures that variable queries have their `refId` set, which is required for Grafana to recognize the result as a variable response. I can't seem to find documentation about this, but it's reflected in other custom variable support implementations: https://github.com/grafana/grafana/blob/15293a2ceb083108c0f15490933db523aa915c7d/public/app/plugins/datasource/loki/components/VariableQueryEditor.tsx#L71

Author: NeedleInAJayStack

src/datasource.ts

@@ -130,6 +130,8 @@ export class HaystackVariableSupport extends CustomVariableSupport<DataSource, H
 
   query(request: DataQueryRequest<HaystackVariableQuery>): Observable<DataQueryResponse> {
     let variableQuery = request.targets[0];
+    // Setting the refId is apparently required for Grafana to recognize the result as a variable
+    variableQuery.refId = 'HaystackVariableQuery';
     let observable = this.onQuery(request);
     return observable.pipe(
       map((response) => {