Currently we always log when we enter _mergeSpills function: https://github.com/mongodb/mongo/blob/deff7f6bf8ed2341856468d20285e03db1460134/src/mongo/db/sorter/sorter_template_defs.h#L695

And in some places we call this function without checking if it is needed, because it is idempotent.

We should only log this if we actually have some work to do because merging spills is expected happen in very rare cases of a realy big sorts (like a new index build of a huge collection).