The metrics_addr option was registered twice: once explicitly and once
implicitly via AddPrefix(libhttp.ConfigInfo, "metrics", ...). Both
pointed at the same MetricsHTTP.ListenAddr field, so options/info
returned a duplicate entry.
Drop the explicit entry and use SetDefault to keep the empty default
(so the metrics server stays off unless configured), matching the
pattern already used for rc_addr.
Fixes#9419