Let scripts/gather-test-memory-data use less memory

What does this MR do and why?

This MRs improves slighly the memory efficiency (lowering max resident set size - see below) of scripts/gather-test-memory-data by streaming content instead of slurping into memory when merging CSVs to ensure constant memory usage.

Screenshots or screen recordings

The resulting report.csv remains identical:

md5sum report.csv tmp/memory_test/report.csv
a3c99010e44c35af051ed856cc33bed6  report.csv
a3c99010e44c35af051ed856cc33bed6  tmp/memory_test/report.csv

Note the maxresident below (35204 ⬇ 15044).

Before

/usr/bin/time ruby <path-to-gdk>/scripts/gather-test-memory-data
0.42user 0.05system 0:00.41elapsed 114%CPU (0avgtext+0avgdata 35204maxresident)k
0inputs+3240outputs (1major+35794minor)pagefaults 0swaps

After

/usr/bin/time ruby <path-to-gdk>/scripts/gather-test-memory-data
0.38user 0.05system 0:00.38elapsed 115%CPU (0avgtext+0avgdata 15044maxresident)k
0inputs+3240outputs (0major+30725minor)pagefaults 0swaps

How to set up and validate locally

• From https://gitlab.com/gitlab-org/gitlab/-/jobs/2039704476
• Download archive https://gitlab.com/gitlab-org/gitlab/-/jobs/2039704476/artifacts/download
• Unzip archive in e.g. /tmp/
• Move already generated tmp/memory_test/report.csv out of the way via mv tmp/memory_test/report.csv .
  • Useful for verifying the output later via md5sum (see above)
• Run /usr/bin/time ruby <path-to-gdk>/scripts/gather-test-memory-data
• Verify that generated tmp/memory_test/report.csv matches report.csv

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.