docs: use test example to document prometheus.InstallNewPipeline

[slagiewka]

After InstallNewPipeline has been updated to return *Exporter only, the example regarding Stop() has not been updated. To properly run Stop() you now have to get to the Controller and pass context.Context.

This PR updates hf to exporter to mirror example/prometheus and make it more obvious - I honestly can't say what hf stands for.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Szymon Łągiewka (72d91d2)

[slagiewka]

@Aneurysm9 done.

One thing that concerns me is that there's already an example test which is not visible in docs. I'm not really familiar with how godoc should handle that so I don't know why it's not there.

[MrAlias]

One thing that concerns me is that there's already an example test which is not visible in docs. I'm not really familiar with how godoc should handle that so I don't know why it's not there.

Those docs are from the old module (there was a versioning issue where we needed a rename). Here are the current ones with the included example.