Service usage metrics

[esune]

There is currently no way - other than post-processing logs maybe - to gather usage metrics for the service.

It should be relatively simple to add usage statistic collection as authentication flows are processed, and store them in the database for future analysis.

Statistics should be high-level quantitative data, and could be used to inspect things such as:

• Number of verifications initiated
• Number of abandoned verifications
• Number of failed verifications
• Number of successful verifications
• Which proof-request configurations are being used (as a detail to the above data)