Replace actor queue size gauge with pair of counters

This is necessary to ensure that when the corresponding actor is overloaded, `/metrics/snapshot` still finishes and not blocked infinitely, or seeing corresponding metrics timeout.

Quoting bmahler from email:

There are instances of Gauges that might be better represented as
Counters. For example, we expose the actor queue sizes using a gauge (known
to be unfortunate!), when instead we could expose two counters for
"enqueued" and "dequeued" messages and infer size from these. We can also
add the ability for callers to manually increment and decrement their
Gauges rather than go through a dispatch.