[Monitoring][Additional-Alerting] Thread Pool Rejections

[igoristic]

Can possible be a single threshold that is applied to most (if not all) currently supported thread pools

Conditions/Threshold Example

Detected 10 search thread pool rejections for the past 5 minutes

Possible troubleshooting actions
NA/TBD

[elasticmachine]

Pinging @elastic/stack-monitoring (Team:Monitoring)