Fix HTTP connection leak upon non-OK status.

Change-Id: Ie7fbd7dcc089b8306b40631be3e3d736c23c1cd3

retrieval/target.go

@@ -246,10 +246,10 @@ func (t *target) scrape(timestamp clientmodel.Timestamp, ingester extraction.Ing
 	if err != nil {
 		return err
 	}
+	defer resp.Body.Close()
 	if resp.StatusCode != http.StatusOK {
 		return fmt.Errorf("server returned HTTP status %s", resp.Status)
 	}
-	defer resp.Body.Close()
 
 	processor, err := extraction.ProcessorForRequestHeader(resp.Header)
 	if err != nil {