New metric: selected/total SocketChannels

[sbordet]

Jetty version
9.4.x

Description
Would be interesting to know the ratio of selected/total SelectionKeys for every select operation.

Chatty protocols such as CometD could have a very high ratio that is unusual in web site applications.

A high ratio could be an indication that a larger number of selectors needs to be configured.

@gregw thoughts?

[stale]

This issue has been automatically marked as stale because it has been a full year without activity. It will be closed if no further activity occurs. Thank you for your contributions.

[sbordet]

This has been implemented in the context of #5787.
In particular, there is now a ManagedSelector.getAverageSelectedKeys() that returns the average number of selected keys for each select().