// round trips (the time it takes to send data to etcd and get the
// complete response back)
//
// If an API request involves N (N>=1) round trips to etcd, then we will sum
// up the latenciy incurred in each roundtrip.

// It uses the context associated with the request in flight, so there
// are no states shared among the requests in flight, and so there is no
// concurrency overhead.