Investigate InstantiatingGrpcChannelProvider's isOnComputeEngine() invocation on every channel creation

[lqiu96]

When a gRPC channel is created, the logic will invoke the canUseDirectPath() method: https://github.com/googleapis/sdk-platform-java/blob/af10a9ef26a5fd9f24fc5341d144b8ee00ff3d00/gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java#L476-L48. This call has a check to isOnComputeEngine() which will read a system file for GCE properties.

We may be able to improve performance a bit as we may only need to read this file once. It may be even more performant for ChannelPools which constantly resize the channels as it'll repeatedly create new channels (and read a file every time).

Consider making the isOnComputeEngine() call non-static and introduce a way to cache the systemProduceName value once it's read the first time. Subsequent calls will not need to read the file and can simply return the cached value.