Implemented host_network_host setting

Signed-off-by: Levi Harrison <git@leviharrison.dev>

discovery/moby/docker.go

@@ -54,6 +54,7 @@ var DefaultDockerSDConfig = DockerSDConfig{
 	RefreshInterval:  model.Duration(60 * time.Second),
 	Port:             80,
 	Filters:          []Filter{},
+	HostNetworkHost:  "localhost",
 	HTTPClientConfig: config.DefaultHTTPClientConfig,
 }
 
@@ -68,6 +69,7 @@ type DockerSDConfig struct {
 	Host            string   `yaml:"host"`
 	Port            int      `yaml:"port"`
 	Filters         []Filter `yaml:"filters"`
+	HostNetworkHost string   `yaml:"host_network_host"`
 
 	RefreshInterval model.Duration `yaml:"refresh_interval"`
 }
@@ -106,6 +108,7 @@ type DockerDiscovery struct {
 	*refresh.Discovery
 	client          *client.Client
 	port            int
+	hostNetworkHost string
 	filters         filters.Args
 }
 
@@ -115,6 +118,7 @@ func NewDockerDiscovery(conf *DockerSDConfig, logger log.Logger) (*DockerDiscove
 
 	d := &DockerDiscovery{
 		port:            conf.Port,
+		hostNetworkHost: conf.HostNetworkHost,
 	}
 
 	hostURL, err := url.Parse(conf.Host)
@@ -245,7 +249,15 @@ func (d *DockerDiscovery) refresh(ctx context.Context) ([]*targetgroup.Group, er
 					labels[model.LabelName(k)] = model.LabelValue(v)
 				}
 
-				addr := net.JoinHostPort(n.IPAddress, strconv.FormatUint(uint64(d.port), 10))
+				// Containers in host networking mode don't have ports,
+				// so they only end up here, not in the previous loop.
+				var addr string
+				if c.HostConfig.NetworkMode != "host" {
+					addr = net.JoinHostPort(n.IPAddress, strconv.FormatUint(uint64(d.port), 10))
+				} else {
+					addr = d.hostNetworkHost
+				}
+
 				labels[model.AddressLabel] = model.LabelValue(addr)
 				tg.Targets = append(tg.Targets, labels)
 			}