Add process measurements for monitoring WiredTiger data handles.

From Atlas documentation https://docs.atlas.mongodb.com/reference/atlas-limits/#collection-and-index-limits :

While there is no hard limit on the number of collections in a single cluster, the performance of a cluster might degrade if it serves a large number of collections and indexes. Larger collections have a greater impact on performance.

but now there is no way to get this information except db.serverStatus()