Stats collection causes unbounded memory growth, slowing RPS over time

[huntharo]

Motivations

• oha memory usage grows about 2 MB every couple of seconds under normal usage conditions at 11k RPS
• After a few minutes, oha slows down to only 8-9k RPS
• Restarting oha, but not the target of the test, restores the 11k RPS rate
• It appears that the increasing memory usage for stats may be causing the reduction in performance over time
• While the requests should not be slowed down by the stats collection, they are going to compete for CPU with the stats processing and the stats processing may be using more CPU as the collection of stats grows larger, which could explain why the RPS rate starts to drop

Slowing RPS Over Time

Restarting oha but not Target Restores RPS

[huntharo]

Related to #361 as the unbounded stats collection is what ultimately caused the OOM kill due to fast looping on failing requests because the underlying connection was disconnected.