chore: release 1.5.1 (#1970)

## Summary
- Override release-please version to 1.5.1 (patch) instead of 1.6.0
- The `feat:` commits for OpenMetrics2 are skeleton code only, not
user-facing features

`Release-As: 1.5.1`

Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>

Author: zeitlinger