[Metricbeat] Add Airflow module in xpack

[paolafrancesca]

Metricbeat Module / Dataset release checklist

This checklist is intended for Devs which create or update a module to make sure modules are consistent.

Modules

For a metricset to go GA, the following criterias should be met:

• Supported versions are documented
• Supported operating systems are documented (if applicable)
• Integration tests exist
• System tests exist
• Automated checks that all fields are documented
• Documentation
• Fields follow ECS and naming conventions
• Dashboards exists (if applicable)
• Kibana Home Tutorial (if applicable)
  • Open PR against Kibana repo with tutorial. Examples can be found here.

Metricbeat module

• Example data.json exists and an automated way to generate it exists (go test -data)
• Test environment in Docker exist for integration tests

[elasticmachine]

Pinging @elastic/obs-dc (Team:Obs-DC)

[paolafrancesca]

• Test environment in Docker exist for integration tests
  I would like to add an integration test but I haven't understood how to setup XPackTest for a lightweight module.