[metric] How long does it take to answer an email in the mailing list?

The goal of this task is to have calculated the time to first response in the mailing lists data source.

As done in other data sources such as Discourse or Issues in GitHub, this number helps to understand the time to attention. However the internal hierarchy of emails and how they have to be calculated makes this analysis a bit more complex than the others.

A potential solution for this would to look for those emails that are starting threads (so they do not have any reply-to mbox header) and take one of them. Then list all of the emails that are referring to this in the reply-to mbox header and take the oldest one. And finally calculate the time difference between these two emails.

This may require the addition of the message_id field to the enriched index.