stats: refactor forEachHostMetric to avoid using clusters()

[adisuissa]

Commit Message: stats: refactor forEachHostMetric to avoid using clusters()
Additional Description:
Currently the forEachHostMetric method invokes clusters which creates a temporary map with all the active and warming clusters, and then the forEachHostMetric method iterates over all the active clusters.
This PR introduces the forEachActiveCluster iterator in the Cluster-Manager that instead of creating the temporary map, allows forEachHostMetric to directly iterate over the active clusters and collect the stats.

Risk Level: low - an internal refactor
Testing: Updated the mocks to support the new iterator.
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A

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Caused by: #44015 was opened by adisuissa.

see: more, trace.