[Fleet] Add support for new template path above data stream folders

[jalvz]

Consider this snippet of an integration's manifest.yml file:

policy_templates:
    inputs:
      - vars:
          - name: host
            type: int

Currently, Kibana substitutes the host variable in every data stream template (data_stream/*/agent/stream). However if there aren't data streams (which will be the case for APM), the configuration above is ignored:

inputs:
  - id: 0e682c50-183a-11eb-916d-71d55143d422
    name: my-apm-integration
    streams: [] # no `host` inside because there aren't streams in the integration
    # more input fields

elastic/package-spec#79 tries to define a new valid template path in <package-name>/<version>/agent/input/ as per discussion.

Kibana would then need to substitute the variables defined at the root in policy_templates in agent/input/template.yml.hbs (I don't have an opinion on what the default template name should be), if such file exists. This would result in a policy like:

inputs:
  - id: 0e682c50-183a-11eb-916d-71d55143d422
    name: my-apm-integration
    streams: []
    host: http://localhost:8200
    # more fields coming from agent/input/template.yml.hbs
    # more input fields

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[ruflin]

I assume the streams: [] part is not needed?

@jen-huang I wonder if we should allow to mix global inputs and local input. My initial gut reaction is that no. Mixing the two might become confusing. If we don't allow to mix it, I would hope it also simplifies the code. Either only global input(s) are used or only data_stream specific inputs. Does this make sense?

[jalvz]

I assume the streams: [] part is not needed?

Right, I don't recall if it is generated or not, but it doesn't matter 👍