Add edits for prometheus module docs

[dedemorton]

A few minor editorial fixes.

@ruflin The fields.yml file for the collector metricset is empty: https://github.com/elastic/beats/blob/master/metricbeat/module/prometheus/collector/_meta/fields.yml. Can you update the fields.yml file (or let me know the fields/descriptions, and I can add them)?

[ruflin]

The fields.yml for collector will stay empty as we don't know the field in advance of the exporter. That is the reason it is a "dyanmic" metricset.

[ruflin]

All the exporters are supported as it is all the same format.

[dedemorton]

@ruflin I think users might be confused if they try look at the "Exported Fields" topic to figure out more about the fields returned by the collector metricset. Could we add some basic content in the fields.yml file so that we end up with a section that says something like the following? At least they'll understand why the field they're looking for is missing from the doc.

collector Fields
Stats about the Prometheus exporter. The fields exported by this metricset vary depending on the exporter that you're using.

[ruflin]

@dedemorton We should not use the fields.yml here as it is also use to generate the template and other things. But we should add this to the docs of the metricset itself.

[dedemorton]

@ruflin Updated based on your feedback.

[ruflin]

@dedemorton LGTM. Needs a rebase because of the other PR's that were merged ...

[dedemorton]

@ruflin Rebased. Should be ready to merge as soon as the checks have run.