docker: add container_id to blkio metrics

all container metrics except blkio have this field

closes #1150

plugins/inputs/docker/README.md

@@ -98,6 +98,7 @@ based on the availability of per-cpu stats on your system.
     - io_serviced_recursive_sync
     - io_serviced_recursive_total
     - io_serviced_recursive_write
+    - container_id
 - docker_
     - n_used_file_descriptors
     - n_cpus

plugins/inputs/docker/docker.go

@@ -328,7 +328,7 @@ func gatherContainerStats(
 		acc.AddFields("docker_container_net", netfields, nettags, now)
 	}
 
-	gatherBlockIOMetrics(stat, acc, tags, now)
+	gatherBlockIOMetrics(stat, acc, tags, now, id)
 }
 
 func calculateMemPercent(stat *types.StatsJSON) float64 {
@@ -356,6 +356,7 @@ func gatherBlockIOMetrics(
 	acc telegraf.Accumulator,
 	tags map[string]string,
 	now time.Time,
+	id string,
 ) {
 	blkioStats := stat.BlkioStats
 	// Make a map of devices to their block io stats
@@ -420,6 +421,7 @@ func gatherBlockIOMetrics(
 	for device, fields := range deviceStatMap {
 		iotags := copyTags(tags)
 		iotags["device"] = device
+		fields["container_id"] = id
 		acc.AddFields("docker_container_blkio", fields, iotags, now)
 	}
 }

plugins/inputs/docker/docker_test.go

@@ -48,6 +48,7 @@ func TestDockerGatherContainerStats(t *testing.T) {
 	blkiofields := map[string]interface{}{
 		"io_service_bytes_recursive_read": uint64(100),
 		"io_serviced_recursive_write":     uint64(101),
+		"container_id":                    "123456789",
 	}
 	acc.AssertContainsTaggedFields(t, "docker_container_blkio", blkiofields, blkiotags)