Add separate host and instance variables.

Change-Id: I9bd4d144b382a7f31ad05c062cb5ba5d04bad067
Closes-Bug: PROD-20985
diff --git a/prometheus/meta/prometheus.yml b/prometheus/meta/prometheus.yml
index 4451e8f..9d2755b 100644
--- a/prometheus/meta/prometheus.yml
+++ b/prometheus/meta/prometheus.yml
@@ -12,7 +12,7 @@
         service: prometheus
       annotations:
         summary: "Prometheus target is down"
-        description: "The Prometheus target for the {{ $labels.job }} job on the {{ $labels.host or $labels.instance }} node is down for 2 minutes."
+        description: "The Prometheus target for the {{ $labels.job }} job on the {{ or $labels.host $labels.instance }} node is down for 2 minutes."
     PrometheusTargetSamplesOrderWarning:
       if: increase(prometheus_target_scrapes_sample_out_of_order_total[1m]) > 0
       labels: