private ManagedChannel getChannel(final Endpoint endpoint, final boolean createIfAbsent) {
        if (createIfAbsent) {
            return this.managedChannelPool.computeIfAbsent(endpoint, this::newChannel);
        } else {
            return this.managedChannelPool.get(endpoint);
        }
    }
GRPC RPC client implement.

@author nicholas.jxf
@author jiachun.fjc
jraft-extension/rpc-grpc-impl/src/main/java/com/alipay/sofa/jraft/rpc/impl/GrpcClient.java