[mongodb] Filter out spans from `findOne`'s internal calls to `find`

[lobsterkatie]

We instrument both the find and findOne methods in our mongodb integration. Because findOne internally calls find, this means that users end up with span trees which look like this:

even though they've only made calls to findOne, not find.

If possible, we should prevent those find spans from showing up.

[AbhiPrasad]

Dup of #4495 - addressed possibly by #4563

[lobsterkatie]

Heh. Not even following our own suggestions! Whoops.

Will close in favor of that one, since it seems to address wider array of problems.