[feature] Make exporting metada optional when exporting the model

[gaocegege]

Is this a BUG REPORT or FEATURE REQUEST?:

Uncomment only one, leave it on its own line:

/kind bug /kind feature

What happened:

Now the metadata will be pulled and stored as orbfile.yaml when we export the model. It should be optional, maybe by flag.

What you expected to happen:

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

[simon-cj]

Only export model directry, not export ormbfile.yaml, right？ I have a problem: the ormbfile.yaml's size is small, why not export, have any other reason？ @gaocegege

[gaocegege]

Some servers will check the mode directory. If it finds that there is a new file, it may fail to serve.

[gaocegege]

It is P2 I think, there is no need to do it now.