[CELEBORN-2463] Add ManuallyExcludedWorkerCount metric to distinguish manually excluded workers

[yew1eb]

What changes were proposed in this pull request?

Add a new master gauge metric ManuallyExcludedWorkerCount that reports the count of workers in the manually excluded list (statusSystem.manuallyExcludedWorkers.size).

The existing ExcludedWorkerCount metric is kept unchanged (still the sum of both excluded and manually excluded workers) for backward compatibility.

Why are the changes needed?

ExcludedWorkerCount counts both workers excluded due to failure reports / unhealthy heartbeats, and workers excluded manually via the /exclude API during maintenance. As a result, alerts based on ExcludedWorkerCount fire during routine maintenance even though the cluster is healthy.

The two sets are already tracked separately internally (excludedWorkers vs manuallyExcludedWorkers) and exposed separately in the /workers API.

With this PR, users can alert on ExcludedWorkerCount - ManuallyExcludedWorkerCount to monitor only failure-driven exclusions.

Does this PR resolve a correctness bug?

• Yes

Does this PR introduce any user-facing change?

• Yes

Add a new master metric ManuallyExcludedWorkerCount.

How was this patch tested?

Compiled the master module locally. The new gauge follows the exact same pattern as the existing ExcludedWorkerCount gauge.

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