v-m: Add host labels to collectd-virt metrics

The *virt* plugin for *collectd* sets `instance` to the name of the
libvirt domain the metric refers to.  This makes it so there is no label
identifying which host the VM is running on.  Thus, if we want to
classify metrics by VM host, we need to add that label explicitly.

Since the `__address__` label is not available during metric relabeling,
we need to store it in a temporary label, which gets dropped at the end
of the relabeling phase.  We copy the value of that label into a new
label, but only for metrics that match the desired metric name.

victoria-metrics/scrape.yml

@@ -80,9 +80,21 @@ scrape_configs:
   - files:
     - /scrape/collectd/scrape-collectd.yml
   relabel_configs:
+  - source_labels:
+    - __address__
+    target_label: host__tmp
   - source_labels: [__address__]
     target_label: __address__
     replacement: '$1:9103'
+  metric_relabel_configs:
+  - source_labels:
+    - __name__
+    - host__tmp
+    separator: ;
+    regex: collectd_virt.*;(.*)
+    target_label: host
+  - action: labeldrop
+    regex: host__tmp
 
 - job_name: sambadc
   scrape_interval: 1m