Add Coverage fuzzing to DevopsAdoption API

What does this MR do?

Adds Coverage fuzzing metric to Devops Adoption API. Relies on index from !64624 (merged)

Migration output

== 20210624112821 AddDevopsAdoptionCoverageFuzzing: reverting =================
-- remove_column(:analytics_devops_adoption_snapshots, :coverage_fuzzing_enabled_count, :integer)
   -> 0.0048s
== 20210624112821 AddDevopsAdoptionCoverageFuzzing: reverted (0.0049s) ========

== 20210624112821 AddDevopsAdoptionCoverageFuzzing: migrating =================
-- add_column(:analytics_devops_adoption_snapshots, :coverage_fuzzing_enabled_count, :integer)
   -> 0.0052s
== 20210624112821 AddDevopsAdoptionCoverageFuzzing: migrated (0.0053s) ========

Query explain

Coverage Fuzzing

 Aggregate  (cost=1976.43..1976.44 rows=1 width=8) (actual time=5.333..5.335 rows=1 loops=1)
   Buffers: shared hit=4077
   I/O Timings: read=0.000 write=0.000
   ->  Merge Semi Join  (cost=0.99..1976.42 rows=1 width=0) (actual time=0.388..5.328 rows=2 loops=1)
         Merge Cond: (projects.id = ci_job_artifacts.project_id)
         Buffers: shared hit=4077
         I/O Timings: read=0.000 write=0.000
         ->  Index Only Scan using idx_projects_on_repository_storage_last_repository_updated_at on public.projects  (cost=0.56..1365.73 rows=654 width=4) (actual time=0.148..3.893 rows=652 loops=1)
               Index Cond: (projects.id = ANY 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               Heap Fetches: 76
               Buffers: shared hit=3298
               I/O Timings: read=0.000 write=0.000
         ->  Index Only Scan using tmp_index on public.ci_job_artifacts  (cost=0.43..606.94 rows=1680 width=4) (actual time=0.060..1.308 rows=664 loops=1)
               Index Cond: ((ci_job_artifacts.file_type = 23) AND (ci_job_artifacts.created_at >= '2021-06-01 00:00:00+00'::timestamp with time zone) AND (ci_job_artifacts.created_at <= '2021-06-30 23:59:59.999999+00'::timestamp with time zone))
               Heap Fetches: 337
               Buffers: shared hit=779
               I/O Timings: read=0.000 write=0.000

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Related to #330389 (closed)