DataProxy: Fix issue overriding response body when response status is 101 (#41364)

When a request going through Grafana data source proxy responds with a websocket 
upgrade response we cannot override the response body since it produces an error. 
This problem seems to have been introduced in Grafana v8.0 by #38962. 
In addition #40303 added same problem.

Fixes #41292

pkg/infra/httpclient/httpclientprovider/datasource_metrics_middleware.go

@@ -96,7 +96,7 @@ func executeMiddleware(next http.RoundTripper, datasourceLabel prometheus.Labels
 			return nil, err
 		}
 
-		if res != nil {
+		if res != nil && res.StatusCode != http.StatusSwitchingProtocols {
 			res.Body = httpclient.CountBytesReader(res.Body, func(bytesRead int64) {
 				responseSizeSummary.Observe(float64(bytesRead))
 			})