net-mgmt/prometheus2: Update 2.15.2 -> 2.16.0

The release adds a few new features, and includes a number of enhancements and bug fixes.

Changelog: https://github.com/prometheus/prometheus/releases/tag/v2.16.0

PR: 244174
Submitted by: David O'Rourke <dor.bsd@xm0.uk> (maintainer)