Merged pull request https://github.com/scylladb/scylla/pull/7018
by Piotr Sarna:
This series addresses various issues with metrics and semaphores - it mainly adds missing metrics, which makes it possible to see the length of the queues attached to the semaphores. In case of view building and view update generation, metrics was not present in these services at all, so a first, basic implementation is added.
More precise semaphore metrics would ease the testing and development of load shedding and admission control.
view_builder: add metrics
db, view: add view update generator metrics
hints: track resource_manager sending queue length
hints: add drain queue length to metrics
table: add metrics for sstable deletion semaphore
database: remove unused semaphore
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