Metrics plugin

Example: metrics

Metrics plugin uses go-metrics to calculate metrics of the server.

It contains some metrics:

1. serviceCounter
2. clientMeter
3. "service_"+servicePath+"."+serviceMethod+"_Read_Qps"
4. "service_"+servicePath+"."+serviceMethod+"_Write_Qps"
5. "service_"+servicePath+"."+serviceMethod+"_CallTime"