Per the FTDC spec:

For a given BSON document, all numeric fields are extracted and converted to uint64_t. Timestamps are extracted as two uint64_t, seconds followed by increment. For doubles, NaN is encoded as zero, doubles less than MIN_INT64 are stored as MIN_INT64, and doubles greater than MAX_INT64 are stored as MAX_INT64.

https://github.com/mongodb/mongo/blob/master/src/mongo/db/ftdc/README.md#extraction

However, it does not appear that the server's FTDC implementation adheres to this, and will actually attempt to encode a double whose value is infinity as infinity.