Extend the existing maglev_info gauge with two more labels:
- version (build version, from -ldflags)
- commit (git short hash)
Mirrors nginx_ipng_stats-plugin's nginx_ipng_info pattern: a
label-only gauge value=1 carrying build metadata for Prometheus
to query and Grafana dashboards to render. Useful for spotting
version drift across the fleet with a single query, and for
populating the "Version" column of the Maglev Nodes table on
the global overview.
Existing label source_tag is preserved.
Plumbing: NewCollector / Register signatures gain version and
commit string parameters; main.go reads them from cmd.Version()
and cmd.Commit() (already imported as 'buildinfo').
pim
d2ee6d009e