[RW2] Return 400 for Exemplars without Series or Histograms not written (#17250)

* fix

Signed-off-by: pipiland2612 <nguyen.t.dang.minh@gmail.com>

* fix cmt

Signed-off-by: pipiland2612 <nguyen.t.dang.minh@gmail.com>

---------

Signed-off-by: pipiland2612 <nguyen.t.dang.minh@gmail.com>

storage/remote/write_handler.go

@@ -429,6 +429,12 @@ func (h *writeHandler) appendV2(app storage.Appender, req *writev2.Request, rs *
 			continue
 		}
 
+		// Validate that the TimeSeries has at least one sample or histogram.
+		if len(ts.Samples) == 0 && len(ts.Histograms) == 0 {
+			badRequestErrs = append(badRequestErrs, fmt.Errorf("TimeSeries must contain at least one sample or histogram for series %v", ls.String()))
+			continue
+		}
+
 		allSamplesSoFar := rs.AllSamples()
 		var ref storage.SeriesRef
 

storage/remote/write_handler_test.go

@@ -418,6 +418,22 @@ func TestRemoteWriteHandler_V2Message(t *testing.T) {
 			expectedCode:     http.StatusBadRequest,
 			expectedRespBody: "parsing labels for series [1 999]: labelRefs 1 (name) = 999 (value) outside of symbols table (size 18)\n",
 		},
+		{
+			desc: "Partial write; TimeSeries with only exemplars (no samples or histograms)",
+			input: append(
+				// Series with only exemplars, no samples or histograms.
+				[]writev2.TimeSeries{{
+					LabelsRefs: []uint32{1, 2},
+					Exemplars: []writev2.Exemplar{{
+						LabelsRefs: []uint32{},
+						Value:      1.0,
+						Timestamp:  1,
+					}},
+				}},
+				writeV2RequestFixture.Timeseries...),
+			expectedCode:     http.StatusBadRequest,
+			expectedRespBody: "TimeSeries must contain at least one sample or histogram for series {__name__=\"test_metric1\"}\n",
+		},
 		{
 			desc: "Partial write; first series with one OOO sample",
 			input: func() []writev2.TimeSeries {