Emit metric when task count is capped by maxTasks

[khandelwal-ayush]

Summary

• Add a numTasksCappedByMaxTasks meter to StickyPartitionAssignmentStrategy that fires whenever the computed task count exceeds maxTasks and gets capped
• Enables alerting on datastreams that are persistently hitting their maxTasks ceiling, which indicates they may need a higher limit or capacity adjustment

Details

When elastic task assignment computes a task count higher than the configured maxTasks, the strategy silently caps it and logs a warning. There was no metric to alert on this condition. This change adds a
per-datastream meter (StickyPartitionAssignmentStrategy..numTasksCappedByMaxTasks) so we can set up alerts for sustained capping, which signals that a datastream is under-provisioned.

Testing Done

Will deploy and test