Title: Add capability for reporting upstream stats per endpoint (backend) from Envoy.

Description:
The upstream load stats from Envoy are aggregated per locality. Global Load balancing using central control plane could provide better balancing with per endpoint stats information.

As the use-case here is for providing better balancing, the per endpoint load report might not be necessary for all servers receiving load report. We probably need some way for the Server to inform the Envoy to send Endpoint granularity load report.

Proposal for change.

https://docs.google.com/document/d/1na2pC2FOfLHhbGyH9VRbO229pJg1LrETUXbbvM8ttOg/edit?usp=sharing