API Changes

See #6472 for details about these changes.

balancer: add StateListener to NewSubConnOptions for SubConn state updates and deprecate Balancer.UpdateSubConnState (#6481)
- UpdateSubConnState will be deleted in the future.
balancer: add SubConn.Shutdown and deprecate Balancer.RemoveSubConn (#6493)
- RemoveSubConn will be deleted in the future.
resolver: remove deprecated AddressType (#6451)
- This was previously used as a signal to enable the "grpclb" load balancing policy, and to pass LB addresses to the policy. Instead, balancer/grpclb/state.Set() should be used to add these addresses to the name resolver's output. The built-in "dns" name resolver already does this.
resolver: add new field Endpoints to State and deprecate Addresses (#6471)
- Addresses will be deleted in the future.

New Features

balancer/leastrequest: Add experimental support for least request LB policy and least request configured as a custom xDS policy (#6510, #6517)
- Set GRPC_EXPERIMENTAL_ENABLE_LEAST_REQUEST=true to enable
stats: Add an RPC event for blocking caused by the LB policy's picker (#6422)

clusterresolver: fix deadlock when dns resolver responds inline with update or error at build time (#6563)
grpc: fix a bug where the channel could erroneously report TRANSIENT_FAILURE when actually moving to IDLE (#6497)
balancergroup: do not cache closed sub-balancers by default; affects rls, weightedtarget and clustermanager LB policies (#6523)
client: fix a bug that prevented detection of RPC status in trailers-only RPC responses when using ClientStream.Header(), and prevented retry of the RPC (#6557)

client & server: Add experimental [With]SharedWriteBuffer to improve performance by reducing allocations when sending RPC messages. (Disabled by default.) (#6309)
- Special Thanks: @s-matyukevich