Improve aggregation of code coverage across jobs

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

At the moment, the pipeline code coverage is an average across all the code coverage that are printed in multiple jobs. https://gitlab.com/gitlab-org/gitlab/blob/master/app/models/ci/pipeline.rb#L669-674

This simple average may not represent the actual code coverage when there are multiple jobs or multiple code bases in the project.

Consider the following scenarios:

A code base has 10 lines, numbered 1-10. The pipeline has 2 jobs:
1. Job 1 has tests covering line 1-7 (70%)
1. Job 2 has tests covering line 1-3 (30%)

Average: 50%
Actual coverage: 70%

A code base has 10 lines, numbered 1-10. The pipeline has 2 jobs:
1. Job 1 has tests covering line 1-3 (30%)
1. Job 2 has tests covering line 5-10 (50%)

Average: 40%
Actual coverage: 80%

A code base has 5 lines of ruby and 5 lines of JS, with 2 jobs:
1. `rspec` has tests covering ruby line 1-5 (100% of Ruby)
1. `jest` has tests covering JS line 1-3 (60% of JS)

Average: 80%
Actual coverage: 100% Ruby, 60% JS

For example, if a code base has 5 lines of ruby and 5 lines of JS, with 3 jobs:
1. `rspec A` has tests covering ruby line 1-2 (40% of Ruby)
1. `rspec B` has tests covering ruby line 4-5 (40% of Ruby)
1. `jest` has tests covering JS line 1-3 (60% of JS)

Average: 46.67%
Actual coverage: 80% Ruby, 60% JS.

Proposal

#367317 (comment 2864287313) - Please help add any suggestions/concerns