Hope this works as CallInvoker in C# does.
The original PR for C# is #5928
CallInvoker is described as to get rid of the tight coupling between MathClient and a "real" Channel. as well as allows intercepting of the calls.

It should work together with #15761.
I will add tests if this PR and #15761 are approved before merging.

The GCP extension will use ChannelInterface and CallInvoker to maintain a channel pool. The implementation and tests drafts can be seen at PR: add GCP extension support via call invoker

Hi @stanley-cheung , please review this API when you have time. Thanks!
This is the only way I find to hook a self-defined channel as well as intercept pre-process and post-process for each RPC naturally.
The purpose of the GCP extension is picking a channel from the pool inside call->start() and save/delete channel from the channel pool inside call->getStatus().

Code sample:

class MyCallInvoker implements Grpc\CallInvoker
{
    ...
}
$call_invoker = new MyCallInvoker();
$client = new Grpc\BaseStub('localhost:50051', [
    'credentials' => Grpc\ChannelCredentials::createInsecure(),
    'grpc_call_invoker' => $call_invoker,
]);