One of the nightly tests failed when asserting on the format of the metrics diff report: https://github.com/FoundationDB/fdb-record-layer/actions/runs/18088622922/attempts/1#summary-51464043202

The test failures are basically from places where it expected the format to be something like - Average change: -2.0 but instead the format was something like - Average change: -2,0 (using a comma as the decimal separator instead of a period).

This is a bit of an interesting one, as unlike a lot of the cases where the answer has been to set the locale to Locale.ROOT explicitly, there could be an argument that this format should respect the user's locale so that users who want numbers formatted like that can get them that way. But it would certainly be easier to just force the format to follow that format, which is what we do in the rest of the codebase.