registerMetrics should catch exceptions

[adamgundry]

While #31 added exception handlers to catch exceptions thrown by updates to Ridley metrics, it doesn't cover exceptions thrown within registerMetrics. Thus if a single RidleyMetric fails to be registered, the entire server thread will be aborted. For example, this arises in conjunction with #39.

I think registerMetrics should be refactored to wrap an exception handler around the registration code for a single metric, so that failures prevent that metric from being registered but do not block others from being registered.