Elasticsearch: Use _field_caps instead of _mapping to get fields (#97607)

pkg/tsdb/elasticsearch/elasticsearch.go

@@ -182,14 +182,21 @@ func (s *Service) getDSInfo(ctx context.Context, pluginCtx backend.PluginContext
 	return &instance, nil
 }
 
+func isFieldCaps(url string) bool {
+	return strings.HasSuffix(url, "/_field_caps") || url == "_field_caps"
+}
+
 func (s *Service) CallResource(ctx context.Context, req *backend.CallResourceRequest, sender backend.CallResourceResponseSender) error {
 	logger := s.logger.FromContext(ctx)
 	// allowed paths for resource calls:
 	// - empty string for fetching db version
 	// - /_mapping for fetching index mapping, e.g. requests going to `index/_mapping`
+	// - /_field_caps for fetching field capabilities, e.g. requests going to `index/_field_caps`
 	// - _msearch for executing getTerms queries
 	// - _mapping for fetching "root" index mappings
-	if req.Path != "" && !strings.HasSuffix(req.Path, "/_mapping") && req.Path != "_msearch" && req.Path != "_mapping" {
+	// - _field_caps for fetching "root" field capabilities
+	if req.Path != "" && !isFieldCaps(req.Path) && req.Path != "_msearch" &&
+		!strings.HasSuffix(req.Path, "/_mapping") && req.Path != "_mapping" {
 		logger.Error("Invalid resource path", "path", req.Path)
 		return fmt.Errorf("invalid resource URL: %s", req.Path)
 	}
@@ -266,6 +273,9 @@ func createElasticsearchURL(req *backend.CallResourceRequest, ds *es.DatasourceI
 	}
 
 	esUrl.Path = path.Join(esUrl.Path, req.Path)
+	if isFieldCaps(req.Path) {
+		esUrl.RawQuery = "fields=*"
+	}
 	esUrlString := esUrl.String()
 	// If the request path is empty and the URL does not end with a slash, add a slash to the URL.
 	// This ensures that for version checks executed to the root URL, the URL ends with a slash.