Currently, NewConcurrencyLimiter essentially hardcodes a
ConcurrencyLimiter to be used for per RPC limiting. But, the concurrency
limiter can be more generalized than that. We will be using it for
pack-objects limiting.

For this reason, we can generalize the constructor and move the specific
per RPC limiting code into WithConcurrencyLimiters, since that is used
specifically with middleware.

Add a NewNullConcurrencyMonitor so we can use it in other packages.

We can also rename newPromMonitor to newPerRPCPromMonitor since it's
specifically creating metrics for RPCs, and the general sounding
newPromMonitor is a bit of a misnomer.