Elasticsearch/Prometheus: Set correct service namespace for SigV4 (#39439) (#39664)

Makes sure to set the SigV4 service namespace for Elasticsearch and Prometheus.

Fixes #38440

(cherry picked from commit 45a844e77e)

pkg/tsdb/elasticsearch/elasticsearch.go

@@ -88,6 +88,11 @@ func newInstanceSettings() datasource.InstanceFactoryFunc {
 			return nil, fmt.Errorf("error getting http options: %w", err)
 		}
 
+		// Set SigV4 service namespace
+		if httpCliOpts.SigV4 != nil {
+			httpCliOpts.SigV4.Service = "es"
+		}
+
 		version, err := coerceVersion(jsonData["esVersion"])
 
 		if err != nil {