As long as it all the data for all metrics fits in RAM, it will work great in MongoDB. Of course, as soon as it doesn't, you're totally hosed.

To expand on the parent, a column store like HBase or Cassandra is perfect as each row can represent a timeslice and then each column can represent a single event or record within that timeslice. As the row gets evicted from it's initial storage in memory, it will be written to disk contiguously, in sorted order, and batch reads of this data is sequential.

It is possible to use MongoDB to store a timeslice as a document, but it is not designed to scale out to store very large numbers of columns within a single document.