Clearer error response from push endpoint when labels are malformed (#1750)

* Clearer error response from push endpoint when labels are malformed

* Concise error formatting

pkg/distributor/distributor_test.go

@@ -67,7 +67,7 @@ func TestDistributor(t *testing.T) {
 			lines:            100,
 			mangleLabels:     true,
 			expectedResponse: success,
-			expectedError:    httpgrpc.Errorf(http.StatusBadRequest, "parse error at line 1, col 4: literal not terminated"),
+			expectedError:    httpgrpc.Errorf(http.StatusBadRequest, "error parsing labels: parse error at line 1, col 4: literal not terminated"),
 		},
 	} {
 		t.Run(fmt.Sprintf("[%d](samples=%v)", i, tc.lines), func(t *testing.T) {

pkg/distributor/validator.go

@@ -59,7 +59,7 @@ func (v Validator) ValidateLabels(userID string, labels string) error {
 		// an orthogonal concept (we need not use ValidateLabels in this context)
 		// but the upstream cortex_validation pkg uses it, so we keep this
 		// for parity.
-		return httpgrpc.Errorf(http.StatusBadRequest, err.Error())
+		return httpgrpc.Errorf(http.StatusBadRequest, "error parsing labels: %v", err)
 	}
 	return cortex_validation.ValidateLabels(v, userID, ls)
 }