monitoring: export Sourcegraph version as metric, render annotation in Grafana on version changes

Created by: bobheadxi

Noticed that cAdvisor has this:

It could be convenient to export/scrape this as a Prometheus Gauge, then we could create an annotation query (random blog post) to have annotations show up in Grafana dashboards when Sourcegraph version changes - could be particularly useful continuously-deployed environments like Cloud, k8s.sgdev.org, etc