Generate fields.yml from prometheus response

[jsoriano]

Parse prometheus response and generate fields for metricbeat light modules.

Questions:

• Do we want something like this?
• It adds hundreds of fields, and we already have the prometheus.metrics.* dynamic mapping, do we want to generate documentation only and not fields?

[exekias]

I think It's great to have docs for all fields, specially if they come for free. As you say, we don't really need the mappings though.

How about doing exactly what's on this PR but add a flag to fields.yml to explicitly say that the field should not end up in mapping?, Something like docs_only for instance