Generate GitLab CI Metrics Reports

GitLab has support for displaying metrics reports in MRs. We should do this. We'll likely need to add a new option to the testsuite/driver/perf_notes.py script to output such a report. It appears that the OpenMetrics format is still under development, but this file format is likely what we need to use. We should consider what information MR participants want to see i.e. expected value and tolerance/deviation.