bug: support array argument to labels() again

[zbjornson]

This was undocumented and untested, and broke in 13.1.0.

_{I'm not sure why we provide 4 ways to set labels.}

Fixes #499

[github-actions]

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[zbjornson]

I'm not sure why we provide 4 ways to set labels

@siimon @SimenB I'm somewhat inclined to not land this. This was undocumented and untyped, and it took 14 months for one user to notice this broke (between the time 13.1 was released and issue was opened). On the other hand, it's easy to support and the work is done. WDYT?

[SimenB]

I'm happy with reducing the API surface (i.e. not land this). However, we can throw an explicit error if an array is passed so the user is aware?

[jdmarshall]

getLabels isn't the hottest path in the code but it's kinda hot.

If I'm reading this correctly, the index.d.ts signatures already exist in master now. Another variation on this is:

  args[0].constructor === Object

This has been sitting this way for a long time now, do we still care?