clientv3/concurrency/election.go
func (e *Election) Proclaim(ctx context.Context, val string) error {
	if e.leaderSession == nil {
		return ErrElectionNotLeader
	}
	client := e.session.Client()
	cmp := v3.Compare(v3.CreateRevision(e.leaderKey), "=", e.leaderRev)
	txn := client.Txn(ctx).If(cmp)
	txn = txn.Then(v3.OpPut(e.leaderKey, val, v3.WithLease(e.leaderSession.Lease())))
	tresp, terr := txn.Commit()
	if terr != nil {
		return terr
	}
	if !tresp.Succeeded {
		e.leaderKey = ""
		return ErrElectionNotLeader
	}

	e.hdr = tresp.Header
	return nil
}
func (e *Election) Proclaim(ctx context.Context, val string) error {
	if e.leaderSession == nil {
		return ErrElectionNotLeader
	}
	client := e.session.Client()
	cmp := v3.Compare(v3.CreateRevision(e.leaderKey), "=", e.leaderRev)
	txn := client.Txn(ctx).If(cmp)
	txn = txn.Then(v3.OpPut(e.leaderKey, val, v3.WithLease(e.leaderSession.Lease())))
	tresp, terr := txn.Commit()
	if terr != nil {
		return terr
	}
	if !tresp.Succeeded {
		e.leaderKey = ""
		return ErrElectionNotLeader
	}

	e.hdr = tresp.Header
	return nil
}
// Proclaim lets the leader announce a new value without another election.