DNSWATCHER_METRICS_USERNAME turns on /metrics, and its Basic Auth used
DNSWATCHER_METRICS_PASSWORD as it was, empty included, so the username
with an empty password got in. config.New now refuses a username
without a password with ErrNoMetricsPassword, whose message names both
variables. The README's variable table says the password is required
with the username.
Judgement call: the check is in New, after buildConfig, since it
compares two settings buildConfig has already read; buildConfig is
also at the linter's length limit.
Model: opus-5-5
sneak