When enable ClusterDiscovery, the count of keeper watch (CurrentMetric_ZooKeeperWatch) will increase indefinitely. It is because ClusterDiscovery will call getNodeNames for each cluster every 2 minites, and it will register a new watch callback object every time, which will lead to a leaking of keeper watches if the cluster never updates. We should always register the same watch callback object instead.

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fix the bug of leaking keeper watches when enable Cluster Discovery.

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

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