Add more etcd metrics to monitor

## Enhancement Task
There are some metrics that may be useful for us to diagnose the problem. See https://etcd.io/docs/v3.4/metrics/etcd-metrics-v3.4.13.txt
Here are some examples:
```
# name: "etcd_mvcc_db_open_read_transactions"
# description: "The number of currently open read transactions"
# type: "gauge"
etcd_mvcc_db_open_read_transactions
```

```
# name: "etcd_server_slow_apply_total"
# description: "The total number of slow apply requests (likely overloaded from slow disk)."
# type: "counter"
etcd_server_slow_apply_total
```