feature: OTLP Export

Add a simple helper function to enable exporting the same Prometheus metrics in OTLP format. This is inline with our goals to support OTel as described here: https://prometheus.io/blog/2024/03/14/commitment-to-opentelemetry/#otlp-export-in-the-ecosystem

This should be a simple helper and I am currently POC-ing it here: https://github.com/grafana/unused/pull/98

