reader_concurrency_sempaphore are triplicated: each metrics is registered
for streaming, user, and system classes.
To fix, just move the metrics registration from database to
reader_concurrency_sempaphore, so each reader_concurrency_sempaphore
instantiated will register its metrics (if its creator asked for it).
Adjust the names given to reader_concurrency_sempaphore so we don't
change the labels.
scylla-gdb is adjusted to support the new names.
7fce057cda