From 3865d69db355f7ba0023bbc72c8d23a062dfad3d Mon Sep 17 00:00:00 2001
From: Maciej Borsz <maciejborsz@google.com>
Date: Thu, 19 Apr 2018 11:42:40 +0200
Subject: [PATCH] etcdserver: add is_leader prometheus metric that is 1 on the
 leader.

Before this change, we had now way to find a leader using /metrics
endpoint. This commit adds a metric to do that.
---
 etcdserver/metrics.go | 7 +++++++
 etcdserver/raft.go    | 5 +++++
 2 files changed, 12 insertions(+)

diff --git a/etcdserver/metrics.go b/etcdserver/metrics.go
index 2b549f738f7..6e95e531af6 100644
--- a/etcdserver/metrics.go
+++ b/etcdserver/metrics.go
@@ -28,6 +28,12 @@ var (
 		Name:      "has_leader",
 		Help:      "Whether or not a leader exists. 1 is existence, 0 is not.",
 	})
+	isLeader = prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: "etcd",
+		Subsystem: "server",
+		Name:      "is_leader",
+		Help:      "Whether or not this member is a leader. 1 if is, 0 otherwise.",
+	})
 	leaderChanges = prometheus.NewCounter(prometheus.CounterOpts{
 		Namespace: "etcd",
 		Subsystem: "server",
@@ -62,6 +68,7 @@ var (
 
 func init() {
 	prometheus.MustRegister(hasLeader)
+	prometheus.MustRegister(isLeader)
 	prometheus.MustRegister(leaderChanges)
 	prometheus.MustRegister(proposalsCommitted)
 	prometheus.MustRegister(proposalsApplied)
diff --git a/etcdserver/raft.go b/etcdserver/raft.go
index 8e46f5e6889..45733512fd1 100644
--- a/etcdserver/raft.go
+++ b/etcdserver/raft.go
@@ -171,6 +171,11 @@ func (r *raftNode) start(rh *raftReadyHandler) {
 
 					atomic.StoreUint64(&r.lead, rd.SoftState.Lead)
 					islead = rd.RaftState == raft.StateLeader
+					if islead {
+						isLeader.Set(1)
+					} else {
+						isLeader.Set(0)
+					}
 					rh.updateLeadership()
 				}