Registered metricsets fail to load if listed as light modules

[jsoriano]

When we added support for light modules in Metricbeat (#12270), we defined that available light metricsets in a module should be listed in a module.yml file.

In the case of mixed modules (modules with light and registered metricsets), only light modules are intended to be listed there. We have found that this is confusing, a developer would expect that metricsets listed there continue working no matter their kind, but current implementation fails to load a light module if one of the metricsets listed in module.yml doesn't have a manifest.yml.

Metricbeat shouldn't try to load metricsets as light metricsets if they are registered, even if they are listed in the module,yml file.

How to reproduce:

• Implement a mixed module
• List all metricsets in the module.yml file
• Configure metricbeat with a light metricset of this module
• Metricbeat fails to load the light metricset

Expected behaviour:

• Metricbeat is able to load the light metricset, ignoring metricsets listed in module.yml that are registered.
• Metricbeat still fails if it is going to load a light module and one of the listed metricsets doesn't have a manifest.yml AND is not registered.

[mtojek]

Done. Resolving.