factored out _make_rest_metrics_server

[zyxue]

continuation of #4268 and #4351 to make main more modular and readable. This is the last PR in this series.

[seldondev]

Hi @zyxue. Thanks for your PR.

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[seldondev]

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[axsaucedo]

/test integration

[axsaucedo]

Thanks for the contribution
/approve