Avoid NPE during metrics execution if the leader is not set on a FOLLOWER node

ago 20, 2021 1:46:28 PM org.apache.catalina.core.StandardWrapperValve invoke
GRAVE: Servlet.service() for servlet [metrics] in context with path [/metrics] threw exception
java.lang.NullPointerException: Cannot invoke "org.apache.zookeeper.server.quorum.Leader.getProposalStats()" because the return value of "org.apache.zookeeper.server.quorum.LeaderZooKeeperServer.getLeader()" is null
        at org.apache.zookeeper.server.quorum.LeaderZooKeeperServer.lambda$registerMetrics$5(LeaderZooKeeperServer.java:122)
        at magnews.zookeeper.ZooKeeperMetricsProviderAdapter$MetricsContextImpl.lambda$registerGauge$0(ZooKeeperMetricsProviderAdapter.java:91)