report: tighten rules on metrics names

report: tighten rules on metrics names

Let's stay close to Varlink's naming rules and insist that metrics
prefixes must be valid varlink interface names, and suffixes are valid
varlink field names.

The former rule is clear: because a metric <x>.<y> can only be provided
by a varlink service <x>, it is obvious we should validate them the
same way. Validating the suffix via varlink field rules is not that
obvious, but I think it makes sense to stay close to Varlink naming
rules if we already started out at one place.