The ServerStatus documentation refers to the following metrics:

• https://docs.mongodb.org/v3.0/reference/command/serverStatus/#serverStatus.storage.freelist.search.bucketExhausted
• https://docs.mongodb.org/v3.0/reference/command/serverStatus/#serverStatus.storage.freelist.search.requests
• https://docs.mongodb.org/v3.0/reference/command/serverStatus/#serverStatus.storage.freelist.search.scanned

However, for each of these, they appear to be missing the word "metrics" - if you look at the actual output from db.serverStatus(), the correct path should be:

• metrics.storage.freelist.search.bucketExhausted
• metrics.storage.freelist.search.requests
• metrics.storage.freelist.search.scanned