Tracer::vecSpans can grow to unbounded size

KV records the time taken to execute various phases of a given operation. This is used to provide visibility into e.g., why an operation was slow.

It was not generally expected that a given operation would record many spans; ops typically have a handful of spans which are interesting enough to record in this manner. However, as a result of other bugs (like MB-56970), KV could attempt to record an unlimited number of spans for a given operation (Tracer/Cookie).

This will lead to unbounded non-bucket memory usage. This could also be problematic if a very large number of spans is logged as part of slow op tracing.

In versions affected by MB-56926, this will not be visible in any KV stats, but will be reflected in memcached RSS.