MINOR: stats: add functions to emit typed fields into a chunk

MINOR: stats: add functions to emit typed fields into a chunk

New function stats_emit_typed_data_field() does exactly like
stats_emit_raw_data_field() except that it also prints the data
type after a colon. This will be used to print using the typed
format.

And function stats_emit_field_tags() appends a 3-letter code
describing the origin, nature, and scope, followed by an optional
delimiter. This will be particularly convenient to dump typed
data.