tests: fix usage of steady_clock in MetricsManager on windows

[MariusBgm]

Subject

Fix sporadic unit test failures on MSVC RelWithDebInfo builds.

Description

Turns out that the calls to steady_clock::now() return the same value sometimes (on Windows!). Those calls are now wrapped and checked against the previous value and it's ensured that the timestamp increases strictly monotonically.

Instructions for review / testing

Developer checklist (address before review)

• [ ] Changelog.md updated
• [ ] Prepared update for depending repositories
• [ ] Documentation updated (public API changes only)
• [ ] API docstrings updated (public API changes only)
• [ ] Rebase → commit history clean
• [ ] Squash and merge → proper PR title

[MariusBgm]

FYI @VDanielEdwards steady_clock::now() is weird on windows in RelWithDebInfo builds. Maybe we should switch to native performance counter APIs on Windows

[VDanielEdwards]

@MariusBgm and @KonradBkd thank you for the fix! I really need to keep clock granularity in mind 😸