Implement OpenTelemetry Metrics hook

[federicobond]

See https://github.com/open-feature/ofep/blob/main/OFEP/metric-hooks.md

[gruebel]

Uh, nice. Would be interested to implement it, but also happy to see newcomer contributions 😄

[danjuv]

I can pick this one up if it's still available