Each server already has a -metricsPort Prometheus listener, but nothing
advertises it, so a central scraper cannot find it. Adds metrics_port to
KeepConnectedRequest and ListClusterNodes (filer, S3, broker), Heartbeat
and DataNodeInfo (volume servers), and GetMasterConfigurationResponse
(masters).
Note the existing metrics_address fields are unrelated: they carry the
Prometheus push gateway that servers push to, not a scrape target.