metrics: 🔖 expose build information to prometheus

[cratelyn]

a somewhat common "best practice" when operating services that expose prometheus metrics is to expose information about the software version, and other build information. this makes dashboards more legible when making queries across long spans of time, and can additionally help with operational tasks like incident response.

we should add a pd_build_info gauge that, upon startup, is set to 1. the label should include the cargo version, and chain id, as a start. other helpful bits of information can be added later, as needed.