Reduce code duplication by defining each metric just once, instead of three times, by having the semaphore register metrics by itself. This also makes the lifecycle of metrics contained in that of the semaphore. This is important on enterprise where semaphores are added and removed, together with service levels.
We don't want all semaphores to export metrics, so a new parameter is introduced and all call-sites make a call whether they opt-in or not.
Fixes: https://github.com/scylladb/scylladb/issues/16402Closesscylladb/scylladb#16383
* github.com:scylladb/scylladb:
database, reader_concurrency_sempaphore: deduplicate reader_concurrency_sempaphore metrics
reader_concurrency_semaphore: add register_metrics constructor parameter
sstables: name sstables_manager
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