report ETCD metrics

[localvar]

Currently, Easegress only reports metrics of itself, but for some issues, like #413, the root cause is on the ETCD side, so it would be helpful for troubleshooting if Easegress can also report ETCD metrics.

[aniaan]

Does the community have plans to support external etcd? In this way, we can use Alibaba Cloud's etcd, which can reduce operation and maintenance costs, and they can handle the reporting of some metrics

[xxx7xxxx]

#1137 fixed