loki: send metadata requests through backend (#48063)

pkg/tsdb/loki/api.go

@@ -25,7 +25,7 @@ func newLokiAPI(client *http.Client, url string, log log.Logger) *LokiAPI {
 	return &LokiAPI{client: client, url: url, log: log}
 }
 
-func makeRequest(ctx context.Context, lokiDsUrl string, query lokiQuery) (*http.Request, error) {
+func makeDataRequest(ctx context.Context, lokiDsUrl string, query lokiQuery) (*http.Request, error) {
 	qs := url.Values{}
 	qs.Set("query", query.Expr)
 
@@ -135,8 +135,8 @@ func makeLokiError(body io.ReadCloser) error {
 	return fmt.Errorf("%v", errorMessage)
 }
 
-func (api *LokiAPI) Query(ctx context.Context, query lokiQuery) (*loghttp.QueryResponse, error) {
+func (api *LokiAPI) DataQuery(ctx context.Context, query lokiQuery) (*loghttp.QueryResponse, error) {
-	req, err := makeRequest(ctx, api.url, query)
+	req, err := makeDataRequest(ctx, api.url, query)
 	if err != nil {
 		return nil, err
 	}
@@ -164,3 +164,41 @@ func (api *LokiAPI) Query(ctx context.Context, query lokiQuery) (*loghttp.QueryR
 
 	return &response, nil
 }
+
+func makeRawRequest(ctx context.Context, lokiDsUrl string, resourceURL string) (*http.Request, error) {
+	lokiUrl, err := url.Parse(lokiDsUrl)
+	if err != nil {
+		return nil, err
+	}
+
+	url, err := lokiUrl.Parse(resourceURL)
+	if err != nil {
+		return nil, err
+	}
+
+	return http.NewRequestWithContext(ctx, "GET", url.String(), nil)
+}
+
+func (api *LokiAPI) RawQuery(ctx context.Context, resourceURL string) ([]byte, error) {
+	req, err := makeRawRequest(ctx, api.url, resourceURL)
+	if err != nil {
+		return nil, err
+	}
+
+	resp, err := api.client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+
+	defer func() {
+		if err := resp.Body.Close(); err != nil {
+			api.log.Warn("Failed to close response body", "err", err)
+		}
+	}()
+
+	if resp.StatusCode/100 != 2 {
+		return nil, makeLokiError(resp.Body)
+	}
+
+	return io.ReadAll(resp.Body)
+}

pkg/tsdb/loki/loki.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"net/http"
 	"regexp"
+	"strings"
 	"sync"
 
 	"github.com/grafana/grafana-plugin-sdk-go/backend"
@@ -25,8 +26,9 @@ type Service struct {
 }
 
 var (
-	_ backend.QueryDataHandler = (*Service)(nil)
+	_ backend.QueryDataHandler    = (*Service)(nil)
-	_ backend.StreamHandler    = (*Service)(nil)
+	_ backend.StreamHandler       = (*Service)(nil)
+	_ backend.CallResourceHandler = (*Service)(nil)
 )
 
 func ProvideService(httpClientProvider httpclient.Provider, tracer tracing.Tracer) *Service {
@@ -89,6 +91,40 @@ func newInstanceSettings(httpClientProvider httpclient.Provider) datasource.Inst
 	}
 }
 
+func (s *Service) CallResource(ctx context.Context, req *backend.CallResourceRequest, sender backend.CallResourceResponseSender) error {
+	url := req.URL
+
+	// a very basic is-this-url-valid check
+	if req.Method != "GET" {
+		return fmt.Errorf("invalid resource method: %s", req.Method)
+	}
+	if (!strings.HasPrefix(url, "/loki/api/v1/label?")) &&
+		(!strings.HasPrefix(url, "/loki/api/v1/label/")) && // the `/label/$label_name/values` form
+		(!strings.HasPrefix(url, "/loki/api/v1/series?")) {
+		return fmt.Errorf("invalid resource URL: %s", url)
+	}
+
+	dsInfo, err := s.getDSInfo(req.PluginContext)
+	if err != nil {
+		return err
+	}
+
+	api := newLokiAPI(dsInfo.HTTPClient, dsInfo.URL, s.plog)
+	bytes, err := api.RawQuery(ctx, url)
+
+	if err != nil {
+		return err
+	}
+
+	return sender.Send(&backend.CallResourceResponse{
+		Status: http.StatusOK,
+		Headers: map[string][]string{
+			"content-type": {"application/json"},
+		},
+		Body: bytes,
+	})
+}
+
 func (s *Service) QueryData(ctx context.Context, req *backend.QueryDataRequest) (*backend.QueryDataResponse, error) {
 	result := backend.NewQueryDataResponse()
 
@@ -129,7 +165,7 @@ func (s *Service) QueryData(ctx context.Context, req *backend.QueryDataRequest)
 
 // we extracted this part of the functionality to make it easy to unit-test it
 func runQuery(ctx context.Context, api *LokiAPI, query *lokiQuery) (data.Frames, error) {
-	value, err := api.Query(ctx, *query)
+	value, err := api.DataQuery(ctx, *query)
 	if err != nil {
 		return data.Frames{}, err
 	}

public/app/plugins/datasource/loki/datasource.ts

@@ -439,8 +439,13 @@ export class LokiDatasource
   }
 
   async metadataRequest(url: string, params?: Record<string, string | number>) {
-    const res = await lastValueFrom(this._request(url, params, { hideFromInspector: true }));
+    if (config.featureToggles.lokiBackendMode) {
-    return res.data.data || res.data.values || [];
+      const res = await this.getResource(url, params);
+      return res.data || res.values || [];
+    } else {
+      const res = await lastValueFrom(this._request(url, params, { hideFromInspector: true }));
+      return res.data.data || res.data.values || [];
+    }
   }
 
   async metricFindQuery(query: string) {