Expose build version in prometheus metrics

[pgporada]

Here's examples of what the metric looks like:

$ curl -sk https://localhost:8080/metrics | grep '^sunlight_build'
sunlight_build_info{commit="4609173f5d638692944d5e66731fec856c9efdfc",version="v0.8.1-0.20260522193737-4609173f5d63+dirty"} 1

The version= information is documented here.