export some metrics for embed etcd

[csuzhangxc]

Feature Request

Is your feature request related to a problem? Please describe:

some /metrics in etcd may be useful to debug the cluster (part of them are documented in https://etcd.io/docs/v3.4.0/metrics/).

Describe the feature you'd like:

export some metrics for embed etcd via DM-master

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[csuzhangxc]

we may also need to export connections error (like TLS issues) vai metrics.