session metrics

Oozie to airflow app session metrics job migration

This job waits for the presence of all partitions in the last 7 days from the webrequest table then computes session related metrics for app pageviews

The job computes the following sessions metrics;

• Number of sessions per user
• Number of pageviews per session
• Session length (gap between first and last pageview, in milliseconds)

The resulting output metrics are stored;

• in output_directory/session_metrics.tsv
• and also exposed through a hive external table at wmf.mobile_apps_session_metrics