Behavior Changes

grpc: Dial("unix://relative-path") no longer works (#4817)
- use "unix://absolute-path" or "unix:relative-path" instead in accordance with our documentation
xds/csds: use new field GenericXdsConfig instead of PerXdsConfig (#4898)
transport: server transport will reject requests with connection header (#4803)

New Features

grpc: support grpc.WithAuthority when secure credentials are used (#4817)
creds/google: add NewDefaultCredentialsWithOptions() to support custom per-RPC creds (#4767, #4830)
authz: create file watcher interceptor for gRPC SDK API (#4760)
attributes: add Equal method (#4855)
resolver: add AddressMap and State.BalancerAttributes (#4855)
resolver: Add URL field to Target to store parsed dial target (#4817)
grpclb: add a target_name field to lb config to specify target when used as a child policy (#4847)
grpclog: support formatting log output as JSON (#4854)

server: add missing conn.Close if the connection dies before reading the HTTP/2 preface (#4837)
grpclb: recover if addresses are received after an empty server list was received previously (#4879)
authz: support empty principals and fix rbac authenticated matcher (#4883)
xds/rds: NACK the RDS response if it contains unknown cluster specifier (#4788)
xds/priority: do not switch to low priority when high priority is in Idle (e.g. ringhash) (#4889)

grpc: stabilize WithDefaultServiceConfig and improve godoc (#4888)
status: clarify FromError docstring (#4880)
examples: add example illustrating the use of unix abstract sockets (#4848)
examples: update load balancing example to use loadBalancingConfig (#4887)
doc: promote WithDisableRetry to stable; clarify retry is enabled by default (#4901)

credentials: Mark TransportCredentials.OverrideServerName method as deprecated (#4817)