Revert regex processor to allow empty values

Resolves #4374

plugins/inputs/docker/docker.go

@@ -366,22 +366,9 @@ func (d *Docker) gatherContainer(
 	var v *types.StatsJSON
 	// Parse container name
 	cname := "unknown"
-	match := false
-	if len(container.Names) == 0 { // for tests
-		match = true
-	}
-
-	for i := range container.Names {
-		if !match {
-			match = d.containerFilter.Match(strings.TrimPrefix(container.Names[i], "/"))
-			if match {
-				cname = strings.TrimPrefix(container.Names[i], "/")
-			}
-		}
-	}
-
-	if !match {
-		return nil
+	if len(container.Names) > 0 {
+		// Not sure what to do with other names, just take the first.
+		cname = strings.TrimPrefix(container.Names[0], "/")
 	}
 
 	// the image name sometimes has a version part, or a private repo
@@ -404,6 +391,10 @@ func (d *Docker) gatherContainer(
 		"container_version": imageVersion,
 	}
 
+	if !d.containerFilter.Match(cname) {
+		return nil
+	}
+
 	ctx, cancel := context.WithTimeout(context.Background(), d.Timeout.Duration)
 	defer cancel()
 	r, err := d.client.ContainerStats(ctx, container.ID, false)
@@ -420,11 +411,6 @@ func (d *Docker) gatherContainer(
 	}
 	daemonOSType := r.OSType
 
-	// use common (printed at `docker ps`) name for container
-	if cname != strings.TrimPrefix(v.Name, "/") && v.Name != "" {
-		tags["container_name"] = strings.TrimPrefix(v.Name, "/")
-	}
-
 	// Add labels to tags
 	for k, label := range container.Labels {
 		if d.labelFilter.Match(k) {
@@ -475,12 +461,12 @@ func (d *Docker) gatherContainer(
 		acc.AddFields("docker_container_health", healthfields, tags, time.Now())
 	}
 
-	parseContainerStats(v, acc, tags, container.ID, d.PerDevice, d.Total, daemonOSType)
+	gatherContainerStats(v, acc, tags, container.ID, d.PerDevice, d.Total, daemonOSType)
 
 	return nil
 }
 
-func parseContainerStats(
+func gatherContainerStats(
 	stat *types.StatsJSON,
 	acc telegraf.Accumulator,
 	tags map[string]string,

plugins/inputs/docker/docker_test.go

@@ -107,7 +107,7 @@ func TestDockerGatherContainerStats(t *testing.T) {
 		"container_image": "redis/image",
 	}
 
-	parseContainerStats(stats, &acc, tags, "123456789", true, true, "linux")
+	gatherContainerStats(stats, &acc, tags, "123456789", true, true, "linux")
 
 	// test docker_container_net measurement
 	netfields := map[string]interface{}{

plugins/processors/regex/regex.go

@@ -67,7 +67,10 @@ func (r *Regex) Apply(in ...telegraf.Metric) []telegraf.Metric {
 	for _, metric := range in {
 		for _, converter := range r.Tags {
 			if value, ok := metric.GetTag(converter.Key); ok {
-				metric.AddTag(r.convert(converter, value))
+				k, v := r.convert(converter, value)
+				if k != "" {
+					metric.AddTag(k, v)
+				}
 			}
 		}
 
@@ -75,11 +78,14 @@ func (r *Regex) Apply(in ...telegraf.Metric) []telegraf.Metric {
 			if value, ok := metric.GetField(converter.Key); ok {
 				switch value := value.(type) {
 				case string:
+					k, _ := r.convert(converter, value)
+					if k != "" {
 						metric.AddField(r.convert(converter, value))
 					}
 				}
 			}
 		}
+	}
 
 	return in
 }