MINOR: stats: create fields types suitable for all CSV output data

MINOR: stats: create fields types suitable for all CSV output data

We're preparing for various data types for each stats field as they
appear in the CSV output. For now we only cover the regular types handled
by printf, so we have 32 and 64 bit ints and counters, strings, and of
course "empty" to indicate that there's nothing in the field and which
guarantees that any accessed entry will return 0.

More types will surely come later so that some fields are properly
represented. For example, we could see limits where only the value 0
doesn't show up, or human time, etc.