Prometheus: Fix fallback http method when no http method specified in jsonData (#65229)

Use POST as default if no http method specified in jsonData

pkg/tsdb/prometheus/resource/resource.go

@@ -30,6 +30,10 @@ func New(
 	}
 	httpMethod, _ := maputil.GetStringOptional(jsonData, "httpMethod")
 
+	if httpMethod == "" {
+		httpMethod = http.MethodPost
+	}
+
 	return &Resource{
 		log:        plog,
 		promClient: client.NewClient(httpClient, httpMethod, settings.URL),