Fix(query-frontend): LokiCodec encode/decode works correctly with lab…

…el values endpoint (#6084)

When encode/decode `http.Request` into `querybase.Request` we re-create request.Path.

Before this PR, both `/labels/` and `/labels/{name}/values` endpoint was decoding into same URL path `/labels/` before
forwarding to the querier. So it always returns label names as response.

This bug became visible after enabling all the middleware for `/labels/{name}/values` endpoint as well via #6072

Signed-off-by: Kaviraj <kavirajkanagaraj@gmail.com>

pkg/querier/queryrange/codec.go

@@ -331,7 +331,7 @@ func (Codec) EncodeRequest(ctx context.Context, r queryrangebase.Request) (*http
 		}
 
 		u := &url.URL{
-			Path:     "/loki/api/v1/labels",
+			Path:     request.Path, // NOTE: this could be either /label or /label/{name}/values endpoint. So forward the original path as it is.
 			RawQuery: params.Encode(),
 		}
 		req := &http.Request{

pkg/querier/queryrange/codec_test.go

@@ -304,6 +304,26 @@ func Test_codec_labels_EncodeRequest(t *testing.T) {
 	require.Equal(t, toEncode.StartTs, req.(*LokiLabelNamesRequest).StartTs)
 	require.Equal(t, toEncode.EndTs, req.(*LokiLabelNamesRequest).EndTs)
 	require.Equal(t, "/loki/api/v1/labels", req.(*LokiLabelNamesRequest).Path)
+
+	// Test labels values endpoint
+	toEncode = &LokiLabelNamesRequest{
+		Path:    "/loki/api/v1/labels/__name__/values",
+		StartTs: start,
+		EndTs:   end,
+	}
+	got, err = LokiCodec.EncodeRequest(ctx, toEncode)
+	require.NoError(t, err)
+	require.Equal(t, ctx, got.Context())
+	require.Equal(t, "/loki/api/v1/labels/__name__/values", got.URL.Path)
+	require.Equal(t, fmt.Sprintf("%d", start.UnixNano()), got.URL.Query().Get("start"))
+	require.Equal(t, fmt.Sprintf("%d", end.UnixNano()), got.URL.Query().Get("end"))
+
+	// testing a full roundtrip
+	req, err = LokiCodec.DecodeRequest(context.TODO(), got, nil)
+	require.NoError(t, err)
+	require.Equal(t, toEncode.StartTs, req.(*LokiLabelNamesRequest).StartTs)
+	require.Equal(t, toEncode.EndTs, req.(*LokiLabelNamesRequest).EndTs)
+	require.Equal(t, "/loki/api/v1/labels/__name__/values", req.(*LokiLabelNamesRequest).Path)
 }
 
 func Test_codec_EncodeResponse(t *testing.T) {