feat: handle query interpolation multi-valued variable (#32)

This is a follow up after PR #31. This makes it possible to query
with both single- and multi-valued variables.

After this PR, the following query in Grafana:
```sql
SELECT * FROM backend WHERE application_name IN ($application) OR duration_ms IN ($duration) OR sub = '$sub'
```

In to:
```sql
SELECT * FROM backend WHERE application_name IN ('Elfskot.Api','Elfsquad.ConfiguratorApi','Elfsquad.OdataApi') OR duration_ms IN ('30', '8') OR sub = '1ff96bd8-53d8-4214-85b9-08da00dc987f'
```

The `WHERE IN (x, y, z)` with single quotes seems to work for both
numbers & strings. The logic in the formatter might need to be improved
for more complex data types.

---------

Signed-off-by: Stan van Rooy <[email protected]>
Signed-off-by: Stan van Rooy <[email protected]>

src/datasource.ts

@@ -59,7 +59,7 @@ export class DataSource extends DataSourceApi<MyQuery, MyDataSourceOptions> {
     const end = range!.to;
 
     const calls = options.targets.map(target => {
-      const query = getTemplateSrv().replace(target.queryText, options.scopedVars);
+      const query = getTemplateSrv().replace(target.queryText, options.scopedVars, this.formatter);
 
       const request = {
         "query": query,
@@ -90,6 +90,13 @@ export class DataSource extends DataSourceApi<MyQuery, MyDataSourceOptions> {
     };
   }
 
+  private formatter(value: string | string[], options: any): string {
+    if (options.multi) {
+      return (value as string[]).map(v => `'${v}'`).join(',');
+    }
+    return value as string;
+  }
+
   async metricFindQuery(query: string, options?: any): Promise<MetricFindValue[]> {
     const to = new Date();
     const from = new Date();