feat: gce metadata for additional interfaces

Signed-off-by: Joey Freeland <joey@free.land>

discovery/gce/gce.go

@@ -178,6 +178,15 @@ func (d *Discovery) refresh(ctx context.Context) ([]*targetgroup.Group, error) {
 			addr := fmt.Sprintf("%s:%d", priIface.NetworkIP, d.port)
 			labels[model.AddressLabel] = model.LabelValue(addr)
 
+			// Append additional interface private IP address metadata if multiple interfaces exist.
+			if len(inst.NetworkInterfaces) > 1 {
+				gceLabelEthAddressPrefix := gceLabel + "private_ip_eth"
+				for idx, iface := range inst.NetworkInterfaces {
+					gceLabelEthAddressPrivate := model.LabelName(fmt.Sprintf("%s%d", gceLabelEthAddressPrefix, idx))
+					labels[gceLabelEthAddressPrivate] = model.LabelValue(iface.NetworkIP)
+				}
+			}
+
 			// Tags in GCE are usually only used for networking rules.
 			if inst.Tags != nil && len(inst.Tags.Items) > 0 {
 				// We surround the separated list with the separator as well. This way regular expressions

docs/configuration/configuration.md

@@ -1131,6 +1131,7 @@ The following meta labels are available on targets during [relabeling](#relabel_
 * `__meta_gce_metadata_<name>`: each metadata item of the instance
 * `__meta_gce_network`: the network URL of the instance
 * `__meta_gce_private_ip`: the private IP address of the instance
+* `__meta_gce_private_ip_eth<int>`: if more than one interface exists
 * `__meta_gce_project`: the GCP project in which the instance is running
 * `__meta_gce_public_ip`: the public IP address of the instance, if present
 * `__meta_gce_subnetwork`: the subnetwork URL of the instance