Report build meta data
Introduces variables (scoped main) for a version, git hash, and build host and
date. These values are passed in at compile-time via the -ldflags argument
from the Makefile.
This build metadata, along with the Go version (ala runtime.Version()), is
both logged at startup, and returned as JSON in /healthz responses.
Finally, a dummy Prometheus gauge (kask_build_information) is exported with
the above metadata as labels, allowing series selection on one or more of these