Move metal-useragent to this repo

[zenorocha]

Is there any reason why https://github.com/metal/metal-useragent isn't here?

[jbalsas]

No reason, we just haven't gotten to it yet. To move a plugin here means we've reviewed it, applied the common guidelines and cleared any standing PRs and issues in the old repo.

We've been doing more or less one every 2 weeks, being metal-position the last one to be included and prioritizing the ones that are requiring active development by our teams.

[diegonvs]

I think we can close this issue, since this package has already been imported.
https://github.com/metal/metal-plugins/tree/master/packages/metal-useragent