Add partition count and top state partition count metrics to InstanceMonitor

[LZD-PratyushBhatt]

Issues

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Currently, Helix provides:

• ResourceMonitor.PartitionGauge - partitions per resource (cluster-wide)
• PerInstanceResourceMonitor.PartitionGauge - partitions per instance per resource

However, there was no way to quickly determine the total partition workload on a single instance across all resources without manually aggregating PerInstanceResourceMonitor metrics.

This PR adds two new instance-level metrics to track partition distribution and workload across Helix instances. These metrics fill a gap in the existing monitoring infrastructure by providing aggregated partition counts at the instance level.

Description

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New Metrics

1. PartitionCount - Total number of partitions assigned to an instance across all resources
2. TopStatePartitionCount - Number of partitions where the instance holds the top state (MASTER in MasterSlave, LEADER in LeaderStandby, etc.)

Tests

Unit tests added: TestInstanceMonitor.java
Manually tested as well on a real Helix cluster. Added/removed the resources to check if the metric is functioning properly.

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Code Quality

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[proud-parselmouth]

LGTM, minor comment

[proud-parselmouth]

nit:can we just do String topState = stateModelDef.getTopState()