'-' is a valid character for tags.
Commit 03e1c7d (module/network: Add link stats, 2022-04-30) introduced
two new tags for the network module: `ul-speed` and `dl-speed`. These
use the `-` character, that was previously never used in any tag.
We had two options: either change those tags to use `_` instead, or just
accept `-`s as a valid character. Going forward, I can see many people
deciding to name their tags with `-` instead of `_`, so I believe it is
better to just accept it once and for all.
Note that `-` cannot be used as the first character of a tag (e.g.
`-tag1`) since the `-` has a special meaning in `.yml` files. I don't
believe this will happen often, however, and should be easy to both
detect and correct if it does.
