metrics: Use the correct term for types

[codebien]

What?

In principle k6 has metric types and users can create custom metrics with them. But the two definitions are not strictly the same.

Checklist

• I have used a meaningful title for the PR.
• I have described the changes I've made in the "What?" section above.
• I have performed a self-review of my changes.
• I have run the npm start command locally and verified that the changes look good.

• I have made my changes in the docs/sources/k6/next folder of the documentation.
• I have reflected my changes in the docs/sources/k6/v{most_recent_release} folder of the documentation.
• I have reflected my changes in the relevant folders of the two previous k6 versions of the documentation (if still applicable to previous versions).

• I have made my changes in the docs/sources/k6/next folder of the documentation.

Related PR(s)/Issue(s)

[CLAassistant]

All committers have signed the CLA.

[codebien]

I should update the all the previous versions + next. I will do it asap tomorrow.

[codebien]

As this is not relevant for the release, I set this back to a draft because it requires additional work.

[joanlopez]

Looks good, despite of what's already being pointed out by Mihail! 👍🏻

[heitortsergent]

@codebien do you want me to finish making the changes for this PR and merging it? 🙇

[codebien]

Hey @heitortsergent, thanks for offering! If you have the time to finish making the changes and merge this PR, that would be great. Otherwise, I will find some time to do it before the end of the week.

[github-actions]

💻 Deploy preview deleted.