CLDR-18356 exporter:(deps): Bump prom-client in /tools/st-prom-exporter

Bumps [prom-client](https://github.com/siimon/prom-client) from 12.0.0 to 15.1.3.
- [Release notes](https://github.com/siimon/prom-client/releases)
- [Changelog](https://github.com/siimon/prom-client/blob/master/CHANGELOG.md)
- [Commits](siimon/prom-client@v12.0.0...v15.1.3)

---
updated-dependencies:
- dependency-name: prom-client
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]