Bump thirdparty/prometheus-cpp from 4b03769 to b123481

Bumps [thirdparty/prometheus-cpp](https://github.com/jupp0r/prometheus-cpp) from `4b03769` to `b123481`.
- [Release notes](https://github.com/jupp0r/prometheus-cpp/releases)
- [Commits](jupp0r/prometheus-cpp@4b03769...b123481)

---
updated-dependencies:
- dependency-name: thirdparty/prometheus-cpp
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>