Document the K6_OTEL_EXPORTER_TYPE variable

[mkooiman]

What?

This PR adds some documentation on the K6_OTEL_EXPORTER_TYPE environment variable which is required in order to use the http exporter type

Checklist

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

1. If updating the documentation for the most recent release of k6:

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

Related PR(s)/Issue(s)

#1805

[CLAassistant]

All committers have signed the CLA.

[heitortsergent]

Thanks for your contribution @mkooiman! 🙇