message: identity: expose OS version metrics
  This expose an info metrics with OS version, which can be thus used
  by external observers to track auto-updates progress across a fleet
  of machine. The final metrics looks like this:
  ```
  zincati_identity_os_version{value="30.20190905.0"} 1
  ```
diff: @@ -5,7 +5,7 @@ use crate::rpm_ostree;
  use failure::{ensure, format_err, Fallible, ResultExt};
  use libsystemd::id128;
  use ordered_float::NotNan;
  -use prometheus::Gauge;
  +use prometheus::{Gauge, IntGaugeVec};
  use serde::Serialize;
  use std::collections::HashMap;
  @@ -22,6 +22,11 @@ lazy_static::lazy_static! {
  "zincati_identity_rollout_wariness",
  "Client wariness for updates rollout"
  ).unwrap();
  + static ref OS_VERSION: IntGaugeVec = register_int_gauge_vec!(
  + "zincati_identity_os_version",
  + "OS version of the booted deployment",
  + &["value"]
  + ).unwrap();
  }
  /// Agent identity.
  @@ -66,6 +71,11 @@ impl Identity {
  id.rollout_wariness = Some(rw);
  }
  + // Export info-metrics with details about booted deployment.
  + OS_VERSION
  + .with_label_values(&[&id.current_os.version])
  + .set(1);
  +
  Ok(id)
  }
  