Added documentation for k6 PR #4634, which adds Pushgateway support

[marcoschulte]

What?

Hi everyone,

This PR adapts the remote write module documentation to describe the changes introduced through grafana/k6#4634

The last "limitation" I've added about using counters and gauges instead of native histograms, and the example about slow requests dominating fast ones, does actually also apply for the remote write case and not just for the Pushgateway. Therefore, it could be debated whether this should be moved up to "Counter and gauges" section.

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)

grafana/k6#4634

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