column_family: More precise count of switched memtables
column_family: More precise count of switched memtables

The memtable_switch_count metric is supposed to count the number of
times a flush has resulted in the memtable being switched out, but we
were incrementing the count regardless of whether we tried to flush an
empty memtable or two or more flushes were coalesced into one. This
patch fixes this by moving the metric to where the memtable is
actually switched.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

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