What would you like to be added?

Currently, etcd has metrics to track the disk I/O latency,

• etcd_disk_wal_fsync_duration_seconds
• etcd_disk_backend_commit_duration_seconds

Also peer-2-peer network round trip,

• etcd_network_peer_round_trip_time_seconds

But from users perspective, they care more about the end-to-end processing duration for each request. So we should add such metrics, and add label something like "requestType", which has the following values,

• range
• put
• delete
• readonly_txn
• txn

Why is this needed?

It's useful to identify performance issue.