Loki: Rename the hint header (#42205)

X-Query-Tag -> X-Query-Tags (https://github.com/grafana/grafana/pull/42108#discussion_r755776672)

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

@@ -188,7 +188,7 @@ describe('LokiDatasource', () => {
         expect(backendSrv.fetch).toBeCalledWith(
           expect.objectContaining({
             headers: {
-              'X-Query-Tag': 'Source=logvolhist',
+              'X-Query-Tags': 'Source=logvolhist',
             },
           })
         );
@@ -200,7 +200,7 @@ describe('LokiDatasource', () => {
         expect(backendSrv.fetch).not.toBeCalledWith(
           expect.objectContaining({
             headers: {
-              'X-Query-Tag': 'Source=logvolhist',
+              'X-Query-Tags': 'Source=logvolhist',
             },
           })
         );

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

@@ -281,7 +281,7 @@ export class LokiDatasource
     }
     const query = this.createRangeQuery(target, options, maxDataPoints);
 
-    const headers = target.volumeQuery ? { 'X-Query-Tag': 'Source=logvolhist' } : undefined;
+    const headers = target.volumeQuery ? { 'X-Query-Tags': 'Source=logvolhist' } : undefined;
 
     return this._request(RANGE_QUERY_ENDPOINT, query, { headers }).pipe(
       catchError((err) => throwError(() => this.processError(err, target))),