Optionally use aggregated VSA backend

What does this MR do and why?

This MR optionally enables the aggregated value stream analytics backend. The change is behind a feature flag (use_vsa_aggregated_tables) that is disabled by default. We plan to try it on gitlab-org since we already aggregated the data for that group.

VSA runs a few different queries:

• Median (implemented in this MR)
• Count (implemented in this MR)
• Average (will be implemented as a follow-up)
• Related records (will be implemented as a follow-up)

The new database queries are using special database tables for querying VSA tables. These are storing eventually consistent, de-normalized data. A high-level table diagram can be seen here: !71279 (merged)

VSA supports filtering the data by several parameters, these have been implemented so the new DB tables can be used:

• date range filter on the start or end event timestamps (&6046 (closed))
• labels
• assignee
• author
• milestone

Implementation

We have a central class that builds VSA queries: Gitlab::Analytics::CycleAnalytics::DataCollector (this will go away at some point). Within this class, we optionally call the new queries by invoking Gitlab::Analytics::CycleAnalytics::Aggregated::DataCollector.

The scopes and the base query builder is tested within the MR. The ee/spec/lib/gitlab/analytics/cycle_analytics/data_collector_spec.rb test files have been modified to test both cases (current and new). This test file runs various high-level tests related to VSA.

How to set up and validate locally

1. Enable the feature

   Feature.enable(:use_vsa_aggregated_tables)

2. Seed a new VSA project

   SEED_CYCLE_ANALYTICS=true SEED_VSA=true FILTER=cycle_analytics rake db:seed_fu

3. The seed script prints the project path, copy it and navigate to the project.
4. Go to the group.
5. Go to Analytics > Value Stream
6. Open the top right dropdown and Create new Value Stream
7. Add a name and save.
8. Start rails console and aggregate the data

   group = Group.find(x)
   Analytics::CycleAnalytics::DataLoaderService.new(group:group, model: Issue).execute
   Analytics::CycleAnalytics::DataLoaderService.new(group:group, model: MergeRequest).execute

9. Load the VSA page again.
10. Inspecting the median and count endpoint requests, we should see that the _stage_events tables are being used.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #335391 (closed)