This Pull Request adds build time informations in a gauge metrics (à la node_exporter) when the prometheus_enabled flag is set to true

# HELP dnscrypt_proxy_build_info A metric with a constant '1' value labeled by version, goversion from which dnscrypt_proxy was built, and the goos and goarch for the build.
# TYPE dnscrypt_proxy_build_info gauge
dnscrypt_proxy_build_info{goarch="amd64" goos="linux" goversion="go1.24.3" version="2.1.12"} 1

Goal for me: be able to ensure latest (or a specific version) of dnscrypt_proxy is running on my machines.

I didn't want to touch the JSON for the "main" Monitoring UI/JSON as this has probably more implications, let me know if you want me to have a look.