Maven virtual registry: backend analytics

🔥 Problem

We need analytics to known how Maven virtual registries are used and the effectiveness of the cache.

🚒 Solution

• Use https://docs.gitlab.com/development/internal_analytics/internal_event_instrumentation/quick_start/ to create an internal event.
• The maven virtual registry has one main endpoint used by package manager clients: pull file. Thus, we will have a single internal event: pull file.
• Additionally, we are interested in known how the pull fill request was fulfilled. Is it using upstreams (no cache) or using the cache? The internal event should have a boolean property: from_cache