Create relevant StatCollectors

[jaeheonshim]

Let's focus on these metrics for now

• Number of users
  • This is the number of users in the users table.
• Number of teams
  • Take care not to count soft-deleted teams
• Number of matches played
• Bytes of student code uploaded
  • Sum all of the file sizes by joining submissions with the file_records table. It's okay to count deleted submissions in the aggregation here, since our platform still processed and played games with that previous submission in the past.

Create StatCollectors for each of these. Make sure they are properly annotated as Component so that Spring can find them. You don't need to create a new file for each StatCollector, define them as default visibility classes within a single java file.

Acceptance Criteria

• GlobalStatsService aggregates all stats listed above periodically and updates the database.