ComputeEngineChannelBuilder has wrong signature

[dapengzhang0]

The generic type in ComputeEngineChannelBuilder class signature

public final class ComputeEngineChannelBuilder
    extends ForwardingChannelBuilder<GoogleDefaultChannelBuilder> 

is wrong, it should be ForwardingChannelBuilder<ComputeEngineChannelBuilder>

However, fixing the bug might be ABI breaking for existing users.

cc @zhangkun83

[dapengzhang0]

Did the following tests:

1. With io.grpc:grpc-alts:1.24.1, the following code can run:

ManagedChannel channel = ComputeEngineChannelBuilder.forTarget(serverAddress)
        .build();

2. With io.grpc:grpc-alts:1.24.1, the following code fails to run:

ManagedChannel channel = ComputeEngineChannelBuilder.forTarget(serverAddress)
       .keepAliveTime(10, TimeUnit.MINUTES).build();

or

ManagedChannelBuilder<GoogleDefaultChannelBuilder> builder =
        ComputeEngineChannelBuilder.forTarget(serverAddress);
ManagedChannel channel = builder.keepAliveTime(10, TimeUnit.MINUTES).build();

Exception in thread "main" java.lang.ClassCastException: io.grpc.alts.ComputeEngineChannelBuilder cannot be cast to io.grpc.alts.GoogleDefaultChannelBuilder
        at io.grpc.examples.alts.HelloWorldAltsClient.run(HelloWorldAltsClient.java:87)
        at io.grpc.examples.alts.HelloWorldAltsClient.main(HelloWorldAltsClient.java:39)

So either no existing users are writing code like above or they have broken code like this but never tested it.

3. Compile with io.grpc:grpc-alts:1.24.1, and run with io.grpc:grpc-alts:1.26.0-SNAPSHOT (with fixed signature) in the classpath:

ManagedChannel channel = ComputeEngineChannelBuilder.forTarget(serverAddress)
        .build();

as well as

ManagedChannelBuilder<?> builder = ComputeEngineChannelBuilder.forTarget(serverAddress);
ManagedChannel channel = builder.keepAliveTime(10, TimeUnit.MINUTES).build();

Both can run.

So we can fix the signature directly without breaking ABI for existing users.