DefaultMetricsHandler returns metrics handler for default Prometheus gatherer/registerer
with logging and continuing on error.
Handler is not protected by HTTP basic authentication - it is done by RunServer.
RunServer runs server for exporter with given name (it is used on landing page) on given address,
with HTTP basic authentication (if configured)
and with given HTTP handler (that should be created with DefaultMetricsHandler or manually).
Function never returns.