table: add metrics for sstable deletion semaphore
table: add metrics for sstable deletion semaphore

It's now possible to read the number of tasks waiting on the
sstable deletion semaphore.

scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla