Add container_id field to docker_container_status metrics (#6019)

plugins/inputs/docker/README.md

@@ -273,6 +273,7 @@ status if configured.
     - container_status
     - container_version
   - fields:
+    - container_id
     - oomkilled (boolean)
     - pid (integer)
     - exitcode (integer)

plugins/inputs/docker/docker.go

@@ -466,6 +466,7 @@ func (d *Docker) gatherContainerInspect(
 			"oomkilled":    info.State.OOMKilled,
 			"pid":          info.State.Pid,
 			"exitcode":     info.State.ExitCode,
+			"container_id": container.ID,
 		}
 
 		finished, err := time.Parse(time.RFC3339, info.State.FinishedAt)

plugins/inputs/docker/docker_test.go

@@ -756,6 +756,32 @@ func TestDockerGatherInfo(t *testing.T) {
 			"container_status":  "running",
 		},
 	)
+
+	acc.AssertContainsTaggedFields(t,
+		"docker_container_status",
+		map[string]interface{}{
+			"container_id": "b7dfbb9478a6ae55e237d4d74f8bbb753f0817192b5081334dc78476296e2173",
+			"exitcode":     int(0),
+			"oomkilled":    false,
+			"pid":          int(1234),
+			"started_at":   int64(1528955333266176036),
+			//"finished_at": float64(0),
+		},
+		map[string]string{
+			"engine_host":       "absol",
+			"container_name":    "etcd2",
+			"container_image":   "quay.io:4443/coreos/etcd",
+			"container_version": "v2.2.2",
+			"ENVVAR1":           "loremipsum",
+			"ENVVAR2":           "dolorsitamet",
+			"ENVVAR3":           "=ubuntu:10.04",
+			"ENVVAR7":           "ENVVAR8=ENVVAR9",
+			"label1":            "test_value_1",
+			"label2":            "test_value_2",
+			"server_version":    "17.09.0-ce",
+			"container_status":  "running",
+		},
+	)
 }
 
 func TestDockerGatherSwarmInfo(t *testing.T) {