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Add TablesToDropQueueSize Metric for Dropped Tables

Add TablesToDropQueueSize metric. It's equal to number of dropped tables, that are waiting for background data removal. #16364 (tavplubix).
Added a new metric TablesToDropQueueSize that represents the number of dropped tables waiting for background data removal.

Why it matters

This feature provides users with insight into the current number of tables that have been dropped but whose data is still pending background deletion, helping monitor and troubleshoot table drop operations and disk space recovery.

How to use it

Users can monitor the TablesToDropQueueSize metric through ClickHouse's existing metrics interface to track how many tables are queued for background data removal. No special configuration is required to enable this metric.