Add heartbeat and zk-connected metrics in defaultMetrics.json

[tapojit047]

Fixes #16148

Description

I have updated https://github.com/apache/druid/blob/master/extensions-contrib/prometheus-emitter/src/main/resources/defaultMetrics.json this file and added the zk-connected and service/heartbeat metrics.

Release note

Improved: Prometheus-emitter extension now emits service/heartbeat and zk-connected metrics.

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This PR has:

• been self-reviewed.
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[kfaraz]

Thanks for the changes, @tapojit047 !!