Resolve "Code Review Analytics MVC - API Implementation"

What does this MR do?

Introduces API for Code Review Analytics

Database performance query plans

explain SELECT  “merge_requests”.* FROM “merge_requests” INNER JOIN “merge_request_metrics” ON “merge_request_metrics”.“merge_request_id” = “merge_requests”.“id” WHERE “merge_requests”.“target_project_id” = 278964 AND (“merge_requests”.“state_id” IN (1)) ORDER BY merge_request_metrics.first_comment_at ASC NULLS LAST LIMIT 20 OFFSET 0

  Limit  (cost=5251.58..5251.63 rows=20 width=731) (actual time=18.736..18.753 rows=20 loops=1)
   Buffers: shared hit=5395
   ->  Sort  (cost=5251.58..5253.50 rows=770 width=731) (actual time=18.734..18.745 rows=20 loops=1)
         Sort Key: merge_request_metrics.first_comment_at
         Sort Method: top-N heapsort  Memory: 59kB
         Buffers: shared hit=5395
         ->  Nested Loop  (cost=0.99..5231.09 rows=770 width=731) (actual time=0.073..17.729 rows=875 loops=1)
               Buffers: shared hit=5392
               ->  Index Scan using idx_merge_requests_on_target_project_id_and_iid_opened on public.merge_requests  (cost=0.43..1349.36 rows=847 width=723) (actual time=0.041..5.514 rows=875 loops=1)
                     Index Cond: (merge_requests.target_project_id = 278964)
                     Buffers: shared hit=1010
               ->  Index Scan using index_merge_request_metrics on public.merge_request_metrics  (cost=0.56..4.57 rows=1 width=12) (actual time=0.013..0.013 rows=1 loops=875)
                     Index Cond: (merge_request_metrics.merge_request_id = merge_requests.id)
                     Buffers: shared hit=4382

Time: 24.041 ms
  - planning: 5.136 ms
  - execution: 18.905 ms
    - I/O read: 0.000 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 5395 (~42.10 MiB) from the buffer pool
  - reads: 0 from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #194161 (closed)