We have observed a bug that caused Scylla to crash due to metrics double
registration. This bug is really difficult to reproduce and was seen
only once in the wild. We think that it may be caused by a request
in-flight keeping a reference to the stats object, making it not
deregister when the index is dropped, which casues a double registration
when we recreate the index, however we are not 100% sure.
This patch makes it so the metrics always get deregistered when we drop the index, which should fix the double registration bug.
Fixes: #27252Closesscylladb/scylladb#28655
579ed6f19f