add segmengt statistics message in datanode and Update documentation

Signed-off-by: XuanYang-cn <xuan.yang@zilliz.com>

docs/developer_guides/chap07_query_service.md

@@ -4,7 +4,7 @@
 
 #### 8.1 Overview
 
-<img src="./figs/query_service.jpeg" width=700>
+<img src="./figs/query_service.png" width=700>
 
 
 

docs/developer_guides/chap09_data_service.md

@@ -190,7 +190,7 @@ type DataNode interface {
   WatchDmChannels(req WatchDmChannelRequest) error
   //WatchDdChannel(channelName string) error
   //SetTimeTickChannel(channelName string) error
-  //SetStatsChannel(channelName string) error
+  //SetStatisticsChannel(channelName string) error
   
   FlushSegments(req FlushSegRequest) error
 }
@@ -218,3 +218,18 @@ type FlushSegRequest struct {
 }
 ```
 
+* *SegmentStatistics*
+
+```go
+type SegmentStatisticsUpdates struct {
+    SegmentID UniqueID
+    MemorySize int64
+    NumRows int64
+}
+
+type SegmentStatistics struct{
+    MsgBase
+    SegStats []*SegmentStatisticsUpdates
+}
+```
+