Fix instance label for es-exporter

Avoid MultipleSeries error in Grafana
Backport: I9a940cc9f2a5dee60de931a59634cdfdec61c903

Change-Id: Icf8e56426c44f72e95707c14f45446874e6f9578
Related-bug: PROD-27904 (PROD:27904)
diff --git a/prometheus/files/server/prometheus.yml b/prometheus/files/server/prometheus.yml
index 055eb69..7611b97 100644
--- a/prometheus/files/server/prometheus.yml
+++ b/prometheus/files/server/prometheus.yml
@@ -384,4 +384,10 @@
       target_label: environment
       replacement: {{ server.config.environment_label }}
     {%- endif %}
+    {%- if endpoint.name in ['remote_agent', 'remote_agent_openstack', 'elasticsearch_exporter'] %}
+    - source_labels: ['__meta_dns_name']
+      regex: ^tasks.(.*)$
+      replacement: $1:{{ endpoint.port }}
+      target_label: 'instance'
+    {%- endif %}
 {%- endfor %}