Server side rejections based on Implicit Request Priority

[tushar-kharbanda72]

Is your feature request related to a problem? Please describe.
#1042 aims to build back-pressure support for Search requests. This feature will be extension to #1180 and #1181 and will be rejecting requests based on implicit priority criteria like fetch phase completion might result in search request completion compared to a query phase etc.

Describe the solution you'd like

With #1180 and #1181, we have the ability to reject and cancel the search requests on the server side based upon point in time resource consumption. This point in-time state should also be considered while admitting the forced tasks, such as retries and fetch operations, as these forced tasks can increase the load further on the nodes.

Back-pressure frame should factor in the point in-time resource consumption for forced search request and take a decision whether to admit the query or not. This will be based on the point in time load Forced will act as priority scheme for the search request but will still not guarantee whether the request will be admitted or not and this will be responsibility of back-pressure framework to decide.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.