stats: allow pre-defined tags

The internal API to create tags has two versions:

1. Create from qualified stat name.
2. Create from stat name plus a set of tags.

We'd like to have a third version:

1. Create from stat name plus a set of tag names. Tag names match against the content of the stat to obtain values. For example, stat "a.b.c" with tag names "d.e" produces "a{d=b,e=c}". Here the separator is a dot, but could be any special character.

The benefit is two-fold:

1. Save on space, instead of "a.d.b.e.c" we can share "d" and "e" symbols between stat names with different tag values.
2. Simplify user experience by not required regexes to achieve the same effect.