Today we expire the client stats for HTTP channels 5 minutes after they
close. It's possible to open a very large number of HTTP channels in 5
minutes, possibly inadvertently, and the stats for those channels can be
overwhelming.
This commit introduces a limit on the number of channels tracked by each
node which applies in addition to the age limit, and makes these limits
configurable via static settings. It drops the pruning of old stats when
starting to track a new channel and instead uses a queue to expire the
oldest stats when each channel closes if necessary to respect the count
limit; it only performs age-based expiry when retrieving the stats,
since the count limit now bounds the memory needed. Finally, it
tightents up some missing synchronization and makes sure that we expose
only immutable objects to the stats subsystem.
1045abe71f