operators: Expose meta information

[EricCousineau-TRI]

To more easily facilitate RobotLocomotion/drake#8389

Cherry-picked from #12

@soonho-tri Would you be OK with reviewing this?

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This change is 

[soonho-tri]

The changes are seemingly harmless, but at the same time we do not utilize info in this PR, yet. But I guess we will do in the future PRs?

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Reviewed 1 of 1 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[EricCousineau-TRI]

Yup, it's used in #12, and I'll use it now in #8389 (hopefully before this PR merges, so it can be validated upstream).

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Review status: all files reviewed at latest revision, all discussions resolved, some commit checks pending.

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Comments from Reviewable

[soonho-tri]

BTW, reviewable website allowed me to review this PR via its interface which is nice. But I guess it's not fully integrated yet as Github still asks me to provide a review.

Do you plan to enable reviewable workflow here?

[soonho-tri]

I'll use it now in #8389

Nice!

[EricCousineau-TRI]

I'm a little tentative to do so (not sure how it will affect TravisCI), but can consider it.

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Review status: all files reviewed at latest revision, all discussions resolved, some commit checks pending.

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Comments from Reviewable

[soonho-tri]

No rush. Just my wish.

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Review status: all files reviewed at latest revision, all discussions resolved, some commit checks pending.

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Comments from Reviewable

[EricCousineau-TRI]

Given that RobotLocomotion/drake#8389 did not even needs this, I'm going to go ahead close this PR.