fix: fix "at most one distinct index is allowed per field" in STRUCT index (#44969)

issue: https://github.com/milvus-io/milvus/issues/42148

Signed-off-by: SpadeA <tangchenjie1210@gmail.com>

internal/datacoord/index_service.go

@@ -59,6 +59,13 @@ func (s *Server) getFieldNameByID(schema *schemapb.CollectionSchema, fieldID int
 			return field.Name, nil
 		}
 	}
+	for _, structField := range schema.GetStructArrayFields() {
+		for _, subField := range structField.GetFields() {
+			if subField.FieldID == fieldID {
+				return subField.Name, nil
+			}
+		}
+	}
 	return "", nil
 }