[BUG] 避免字符串和MetricNode对象的转换,减少内存占用

https://github.com/alibaba/Sentinel/blob/195150bc745927429e9f14f501907310b46d702f/sentinel-core/src/main/java/com/alibaba/csp/sentinel/node/metric/MetricsReader.java#L54-L56

55行没必要为了获取时间戳而将字符串转成``MetricNode``对象, 直接通过字符串截取就行,
```java
int timestampIndex = s.indexOf("|");
String timestamp = Long.parseLong(s.substring(0, timestampIndex));
```
https://github.com/alibaba/Sentinel/blob/195150bc745927429e9f14f501907310b46d702f/sentinel-transport/sentinel-transport-common/src/main/java/com/alibaba/csp/sentinel/command/handler/SendMetricCommandHandler.java#L97-L100
SendMetricCommandHandler中为了得到`ThinString`, 又将`MetricNode`转成字符串, `MetricNode`成了冗余转换, 会增加内存占用
不如直接采用`FatString`字符串分割
```java
int timestampIndex = s.indexOf("|");
String timestamp = s.substring(0, timestampIndex);

int i = s.indexOf("|", timestampIndex + 20);
String metric = s.substring(i);

System.out.println(timestamp + metric);
```