Fix miss-named metrics flag/config

Description

The current config/flag for controlling the Prometheus metrics listener is named "metrics server". This is confusing for users since they think this is the destination address of the metrics (push), rather than the listening address.

Proposal

Rename "metrics server" to "listen address" to follow standard Go/Prometheus naming conventions for metrics/pprof listeners.

Links to related issues and merge requests / references

See: https://groups.google.com/forum/#!msg/prometheus-users/BnigkKfnatA/_Ny1Kdz0AAAJ