New Features

Netty client: Connection Keepalives now work.
Keepalives in Netty and OkHttp now allow sending pings without outstanding RPCs. The minimum keepalive time was also reduced from 1 minute to 10 seconds. Clients must get permission from the services they use before enabling keepalive.
Netty server: now detects overly aggressive keepalives from clients, with configurable limits. Defaults to permitting keepalives every 5 minutes only while there are outstanding RPCs, but clients must not depend on this value.
Netty server: can now use keepalives to detect dead connections
Netty server: can now enforce a maximum connection age. This is useful when using pick-first or behind a L4 load balancer
Netty client: can now use HTTP GET for safe and idempotent RPCs. Netty server does not yet accept these RPCs. This is experimental with planned incompatible wire format changes
Added io.grpc.protobuf.StatusProto for working with com.google.rpc.Status, which can include additional error details
Introduced StreamTracers on client- and server-side. These can be used by tracing frameworks and LoadBalancers to be notified of relevant RPC events and details. OkHttp and Netty support the tracers, but InProcess does not

API Changes

MethodDescriptor.Builder is now non-experimental
The alias from LoadBalancer2 to LoadBalancer was removed as planned
LoadBalancer.SubchannelPicker.pickSubchannel(Attributes, Metadata) was deleted as planned in favor of the more extensible LoadBalancer.SubchannelPicker.pickSubchannel(PickSubchannelArgs)
CallOptions.withAffinity is now deprecated in favor of withOption and will be removed in the next release
NettyChannelBuilder. and OkHttpChannelBuilder.enableKeepAlive is now deprecated in favor of keepAliveTime and keepAliveTimeout and will be removed in the next release
ResolvedServerInfoGroup and ResolvedServerInfo are deprecated and are planned for deletion in the next release

The Netty transport now handles the Netty Channel failing to be constructed by cleanly reporting the error. Previously this would cause a NullPointerException. This is generally useful for debugging broken shading configuration
OkHttp: Fix compatibility issue with newer versions of Conscrypt