To improve debuggability of `agreement among raft nodes before
linearized reading`, we added some tracing inside
`linearizableReadLoop`.
This will allow us to know the timing of `s.r.ReadIndex` vs
`s.applyWait.Wait(rs.Index)`.
SubTraceStart and SubTraceEnd steps are only placeholders, not really
steps, we should skip them when logging the long duration steps,
otherwise these steps will lead to incorrect start time and duration
of subsequent steps.
* etcdserver: add trace for txn request
* pkg/traceutil: added StopSubTrace as a sign of the end of subtrace. Added test case for logging out subtrace.
steps:range from the in-memory index tree; range from boltdb.
etcdserver: add tracing steps: agreement among raft nodes before
linerized reading; authentication; filter and sort kv pairs; assemble
the response.
