message: cincinnati: record error-kind in metrics
  This augments the Cincinnati-related error metric with the machine-friendly
  kind. The resulting metrics now looks like this:
  ```
  zincati_cincinnati_update_checks_errors_total{kind="generic_http_418"} 42
  ```
diff: @@ -13,7 +13,7 @@ use crate::rpm_ostree::Release;
  use failure::{bail, Fallible};
  use futures::future;
  use futures::prelude::*;
  -use prometheus::{IntCounter, IntGauge};
  +use prometheus::{IntCounter, IntCounterVec, IntGauge};
  use serde::Serialize;
  use std::collections::BTreeSet;
  @@ -39,10 +39,11 @@ lazy_static::lazy_static! {
  "zincati_cincinnati_update_checks_total",
  "Total number of checks for updates to the upstream Cincinnati server."
  )).unwrap();
  - static ref UPDATE_CHECKS_ERRORS: IntCounter = register_int_counter!(opts!(
  + static ref UPDATE_CHECKS_ERRORS: IntCounterVec = register_int_counter_vec!(
  "zincati_cincinnati_update_checks_errors_total",
  - "Total number of errors on checks for updates."
  - )).unwrap();
  + "Total number of errors while checking for updates.",
  + &["kind"]
  + ).unwrap();
  }
  /// Cincinnati configuration.
  @@ -88,7 +89,7 @@ impl Cincinnati {
  log::trace!("checking upstream Cincinnati server for updates");
  let update = self.next_update(id, deployments).map_err(|e| {
  - UPDATE_CHECKS_ERRORS.inc();
  + UPDATE_CHECKS_ERRORS.with_label_values(&[&e.error_kind()]).inc();
  log::error!("failed to check Cincinnati for updates: {}", e)
  });
  Box::new(update)
  