Add meta arg in grpc_predict_seldon()

[ysk24ok]

What this PR does / why we need it:

SSIA

rest_predict_seldon(), rest_predict_gateway() and grpc_predict_gateway() have meta argument, but only grpc_predict_seldon() doesn't have it.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[seldondev]

Hi @ysk24ok. Thanks for your PR.

I'm waiting for a SeldonIO or todo member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the jenkins-x/lighthouse repository.

[ukclivecox]

/ok-to-test

[ysk24ok]

@cliveseldon Could you review again?