`ConnCounter` tracks connection counts by manually updating a gauge.
This approach can lead to negative connection counts when gauges expire.
The gauges used in this class expire 15 mins after the last update.
Stable http/2 connections to origins are especially susceptible to
getting lost.

The new approach relies on a PolledMeter monitoring an AtomicLong. Each
event loop independently tracks its own counts (internally gauges
maintain a list of all monitored numbers with the same id, and sum them
up). I did this to simplify the cleanup logic