rename inference_model metrics to inference_objective

[JeffLuoo]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Rename metric prefixed with inference_model to inference_objective.

Which issue(s) this PR fixes:

Fixes #1566

Does this PR introduce a user-facing change?:

rename metric prefix from `inference_model` to `inference_objective`.

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[kfswain]

This LGTM from the standpoint of a rename. We can probably take a longer term AI on if we want to reshape the metrics we output. But I'll leave that to you @JeffLuoo to decide when/how

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JeffLuoo, kfswain

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