Measure normalized time to fill forms by respondents and track on a dashboard

[amitogp]

Starting point: Form renders
End point: Clicks submit

Normalization: Time should be calculated per 10 visible fields

@timotheeg please add your idea about ensuring visible fields

[timotheeg]

The submission rest end point should probably contain an additional metadata block.

It's not great to trust frontend data, but in practice, it will likely be OK:
TODO:

• frontend to track start time on load
• frontend to track time to where submit is pressed
• frontend to report in metadata: submissionTime and numVisibleFields
• On successful acceptance of a submission, server to read the metadata and report 2 distribution metrics:
  • formsg.submission.responseTime
  • formsg.submission.normalizedResponseTime which is basically 10 * submissionTime / numVisibleFields
• metadata to be sent in the clear (system should work even in storage mode)
• (optional): the metadata can be stored in the submission object if we want to run queries in mongo later on, or if we want to productize the data to present to admins how much time respondent really spend on the form.

Important note:
By tracking the metric in DataDog, we can immediately check if the response time is stable. We cannot howver track the response time by form individually since we cannot add form ids in DD tags.