Log Message:
grpc py-grpcio py-grpcio-testing py-grpcio-tools: updated to 1.42.0

Release v1.42.0

Core

Update RDS parsing for use on servers.
Upgrade Abseil to LTS 20210324, Patch 2.
Upgrade bazel to 4.2.1 (LTS), upgrade bazel toolchain to 4.1.0.
Remove old backwards compatibility cronet compression workaround code.
EventEngine Test Suite: Timers.
EventEngine::Closure.
OpenCensusCallTracer: Move context generation to StartTransportStreamOpBatch.
Fix client idle filter.
allow connectivity state watching to work on lame channels.
grpclb: implement subchannel caching.
xds: change CSDS to populate new generic_xds_configs field.

C++

Describe support-levels for undocumented supported platforms.

C#

Fix link error when building app with Xamarin.iOS.
C#: metadata.Get and GetAll should accept uppercase keys.
Fix use-after-free metadata corruption in C# when receiving response headers for \ 
streaming response calls.

Objective-C

[objc] GRPCErrorCode enum base type to int32_t.
[objc] Adding lightweight generic to GPRCCallOptions's initialMetadata prop .
[objc] GRPCMetadataDictionary convenient typedef.
[objc] Switch to proto forward declare for gRPC codegen plugin.

Python

Add Aspects to Bazel py_proto_library and py_grpc_library Rules.
[Aio] Add add_done_callback/done/cancelled methods to ServicerContext.
[Aio] Correct the typing of input metadata.
Address leak when using request stream interceptors
Catch ExecuteBatchError in _consume_request_iterator.
[Aio] Resolve deprecated warnings from asyncio.
Create Bazel gevent test harness.
Add python_requires >=3.6 to grpcio-* packages.
fix: use == instead of is when comparing with a certain types of literals.
python: fix type annotation for the _metadata field.

Ruby

ruby: add arm64 darwin support.
ruby: build native Darwin gems using rake-compiler-dock.