[kudu-CR] WIP: KUDU-1506 Add consensus lag metrics

["Andrew Wong (Code Review)" <ge...@cloudera.org>]

Hello Kudu Jenkins,

I'd like you to reexamine a change.  Please visit

    http://gerrit.cloudera.org:8080/6451

to look at the new patch set (#6).

Change subject: WIP: KUDU-1506 Add consensus lag metrics
......................................................................

WIP: KUDU-1506 Add consensus lag metrics

It may be useful to see how far behind the leaders followers are with
regard to replicating operations. As such, this patch adds a metric
facilitate this.

The new metric counts the number of operations behind the maximum
applied operation, as seen by the leader, each peer is. It is updated
when a replica receives a request from a leader, and every time the
peer queue updates its metrics, i.e. after every append on a follower
and after parsing responses from peers on a leader.

This currently still needs to address using time as a lag metric, as
per the JIRA.

Change-Id: Ida8e992cc2397ca8d5873e62961a65f618d52c36
---
M src/kudu/consensus/consensus.proto
M src/kudu/consensus/consensus_queue-test.cc
M src/kudu/consensus/consensus_queue.cc
M src/kudu/consensus/consensus_queue.h
M src/kudu/consensus/raft_consensus.cc
5 files changed, 54 insertions(+), 6 deletions(-)


  git pull ssh://gerrit.cloudera.org:29418/kudu refs/changes/51/6451/6
-- 
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Gerrit-MessageType: newpatchset
Gerrit-Change-Id: Ida8e992cc2397ca8d5873e62961a65f618d52c36
Gerrit-PatchSet: 6
Gerrit-Project: kudu
Gerrit-Branch: master
Gerrit-Owner: Andrew Wong <aw...@cloudera.com>
Gerrit-Reviewer: Andrew Wong <aw...@cloudera.com>
Gerrit-Reviewer: David Ribeiro Alves <dr...@apache.org>
Gerrit-Reviewer: Kudu Jenkins
Gerrit-Reviewer: Mike Percy <mp...@apache.org>
Gerrit-Reviewer: Todd Lipcon <to...@apache.org>