Add rate limiting and monitoring for POSIXv2 #3058
Description
In discussions with LIGO, the top of the punchlist for putting POSIXv2 into production is the addition of rate limiting (a-la XrdThrottle) and Prometheus metrics.
The Prometheus metrics are at least a touch tricky -- the existing ones all have xrootd_ prefixes even though xrootd is not present in POSIXv2! The plan is to create a compatibility layer: have a new generic name for metrics and have xrootd publish to both old and new.