discovery/kubernetes: add container image as metadata

This commits adds a __meta_kubernetes_pod_container_image as a new
metadata label. This can be used to alert on mismatched versions of
targets who don't have a build_info metric, as well as injecting it into
log lines for other consumers of discovery/kubernetes (e.g., Promtail).

Signed-off-by: Robert Fratto <robertfratto@gmail.com>

discovery/kubernetes/endpoints.go

@@ -322,6 +322,7 @@ func (e *Endpoints) buildEndpoints(eps *apiv1.Endpoints) *targetgroup.Group {
 					ports := strconv.FormatUint(uint64(port.Port), 10)
 
 					target[podContainerNameLabel] = lv(c.Name)
+					target[podContainerImageLabel] = lv(c.Image)
 					target[podContainerPortNameLabel] = lv(cport.Name)
 					target[podContainerPortNumberLabel] = lv(ports)
 					target[podContainerPortProtocolLabel] = lv(string(port.Protocol))
@@ -380,6 +381,7 @@ func (e *Endpoints) buildEndpoints(eps *apiv1.Endpoints) *targetgroup.Group {
 				target := model.LabelSet{
 					model.AddressLabel:            lv(a),
 					podContainerNameLabel:         lv(c.Name),
+					podContainerImageLabel:        lv(c.Image),
 					podContainerPortNameLabel:     lv(cport.Name),
 					podContainerPortNumberLabel:   lv(ports),
 					podContainerPortProtocolLabel: lv(string(cport.Protocol)),

discovery/kubernetes/endpointslice.go

@@ -350,6 +350,7 @@ func (e *EndpointSlice) buildEndpointSlice(eps endpointSliceAdaptor) *targetgrou
 					ports := strconv.FormatUint(uint64(*port.port()), 10)
 
 					target[podContainerNameLabel] = lv(c.Name)
+					target[podContainerImageLabel] = lv(c.Image)
 					target[podContainerPortNameLabel] = lv(cport.Name)
 					target[podContainerPortNumberLabel] = lv(ports)
 					target[podContainerPortProtocolLabel] = lv(string(cport.Protocol))
@@ -398,6 +399,7 @@ func (e *EndpointSlice) buildEndpointSlice(eps endpointSliceAdaptor) *targetgrou
 				target := model.LabelSet{
 					model.AddressLabel:            lv(a),
 					podContainerNameLabel:         lv(c.Name),
+					podContainerImageLabel:        lv(c.Image),
 					podContainerPortNameLabel:     lv(cport.Name),
 					podContainerPortNumberLabel:   lv(ports),
 					podContainerPortProtocolLabel: lv(string(cport.Protocol)),

discovery/kubernetes/pod.go

@@ -177,6 +177,7 @@ const (
 	podNameLabel                  = metaLabelPrefix + "pod_name"
 	podIPLabel                    = metaLabelPrefix + "pod_ip"
 	podContainerNameLabel         = metaLabelPrefix + "pod_container_name"
+	podContainerImageLabel        = metaLabelPrefix + "pod_container_image"
 	podContainerPortNameLabel     = metaLabelPrefix + "pod_container_port_name"
 	podContainerPortNumberLabel   = metaLabelPrefix + "pod_container_port_number"
 	podContainerPortProtocolLabel = metaLabelPrefix + "pod_container_port_protocol"
@@ -266,9 +267,10 @@ func (p *Pod) buildPod(pod *apiv1.Pod) *targetgroup.Group {
 			// We don't have a port so we just set the address label to the pod IP.
 			// The user has to add a port manually.
 			tg.Targets = append(tg.Targets, model.LabelSet{
-				model.AddressLabel:    lv(pod.Status.PodIP),
+				model.AddressLabel:     lv(pod.Status.PodIP),
-				podContainerNameLabel: lv(c.Name),
+				podContainerNameLabel:  lv(c.Name),
-				podContainerIsInit:    lv(strconv.FormatBool(isInit)),
+				podContainerImageLabel: lv(c.Image),
+				podContainerIsInit:     lv(strconv.FormatBool(isInit)),
 			})
 			continue
 		}
@@ -280,6 +282,7 @@ func (p *Pod) buildPod(pod *apiv1.Pod) *targetgroup.Group {
 			tg.Targets = append(tg.Targets, model.LabelSet{
 				model.AddressLabel:            lv(addr),
 				podContainerNameLabel:         lv(c.Name),
+				podContainerImageLabel:        lv(c.Image),
 				podContainerPortNumberLabel:   lv(ports),
 				podContainerPortNameLabel:     lv(port.Name),
 				podContainerPortProtocolLabel: lv(string(port.Protocol)),