Add API call to increment Redis HLL counter on user_id and metric

Problem to solve

Allow to track UI events

Proposal

• Add an API call that takes user_id and a metric key
• Each time this is called increment a Redis HLL counter for that key
• Restrict, control and validated key names in order to not get malicious informations

Details

Use case for tracking how many users clicked on Expand button for checking vulnerabilities

See note #232831 (comment 393434345)

Notes/Questions

• Should we have each metric under a specific feature flag?

Resources

https://docs.gitlab.com/ee/api/api_resources.html

cc @gitlab-org/growth/telemetry/engineers