Handle unknown performance counters #1534
Description
We currently support 3 perf counter providers (System.Runtime, Microsoft-AspNetCore-Server-Kestrel, Npgsql), with some hardcoded metadata definitions that determine what we do with each counter (max, average...). Make this work without hard-coded metadata, possibly by just emitting a set of fixed reduce operations (either on all counters, or on unknown ones).
See #1529