build(deps): bump prometheus from 0.13.3 to 0.13.4

Bumps [prometheus](https://github.com/tikv/rust-prometheus) from 0.13.3 to 0.13.4.
- [Changelog](https://github.com/tikv/rust-prometheus/blob/master/CHANGELOG.md)
- [Commits](tikv/rust-prometheus@v0.13.3...v0.13.4)

---
updated-dependencies:
- dependency-name: prometheus
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>