✨ Handle multiple runs of linter and coverage in metric report

[ckunki]

This ticket needs to be fixed before 2024 Nov 30, see https://github.com/actions/upload-artifact.

Currently, only a single file .lint.txt or .coverage is evaluated in metrics.py.

This also leads to problems with github actions/upload-artifact and actions/download-artifact which have slightly different semantics in @v4 vs. @v3. Additionally, actions upload- and download- must be using the same version.

The current implementation additionally is inconsistent, as the single file may be randomly selected.

The current ticket proposes to

• to ensure unique filenames by adding a prefix for the matrix build coordinate, e.g. python version
• enhance the report to
  • Merge the coverage of all runs for all matrix build coordinates
  • Create an aggregated version of the linter results, e.g. average, max, or min score.
• Merging is also required when separating different classes of tests, e.g. fast vs. slow tests, see https://github.com/exasol/saas-api-python/issues/53

[tkilias]

In a first version, the linter reports could be merged by having a section for each matrix build coordinates