some low level tests, like the ones exercising sstables, creates
multiple tables. and we are going to add per-table metrics and
the new metrics uses the ks.cf as part of its unique id. so,
once the per-table metrics is enabled, the sstable tests would fail.
as the metrics subsystem does not allow registering multiple
metric groups with the same name.
so, in this change, we add a new constructor for `simple_schema`,
so that we can customize the the schema's ks and cf when creating
the `simple_schema`. in the next commit, we will use this new
constructor in a sstable test which creates multiple tables.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
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