Add clone processor (#6529)

plugins/processors/clone/README.md

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+# Clone Processor Plugin
+
+The clone processor plugin create a copy of each metric passing through it,
+preserving untouched the original metric and allowing modifications in the
+copied one.
+
+The modifications allowed are the ones supported by input plugins and aggregators:
+
+* name_override
+* name_prefix
+* name_suffix
+* tags
+
+Select the metrics to modify using the standard
+[measurement filtering](https://github.com/influxdata/telegraf/blob/master/docs/CONFIGURATION.md#measurement-filtering)
+options.
+
+Values of *name_override*, *name_prefix*, *name_suffix* and already present
+*tags* with conflicting keys will be overwritten. Absent *tags* will be
+created.
+
+A typical use-case is gathering metrics once and cloning them to simulate
+having several hosts (modifying ``host`` tag).
+
+### Configuration:
+
+```toml
+# Apply metric modifications using override semantics.
+[[processors.clone]]
+  ## All modifications on inputs and aggregators can be overridden:
+  # name_override = "new_name"
+  # name_prefix = "new_name_prefix"
+  # name_suffix = "new_name_suffix"
+
+  ## Tags to be added (all values must be strings)
+  # [processors.clone.tags]
+  #   additional_tag = "tag_value"
+```

plugins/processors/clone/clone.go

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+package clone
+
+import (
+	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/plugins/processors"
+)
+
+var sampleConfig = `
+  ## All modifications on inputs and aggregators can be overridden:
+  # name_override = "new_name"
+  # name_prefix = "new_name_prefix"
+  # name_suffix = "new_name_suffix"
+
+  ## Tags to be added (all values must be strings)
+  # [processors.clone.tags]
+  #   additional_tag = "tag_value"
+`
+
+type Clone struct {
+	NameOverride string
+	NamePrefix   string
+	NameSuffix   string
+	Tags         map[string]string
+}
+
+func (c *Clone) SampleConfig() string {
+	return sampleConfig
+}
+
+func (c *Clone) Description() string {
+	return "Clone metrics and apply modifications."
+}
+
+func (c *Clone) Apply(in ...telegraf.Metric) []telegraf.Metric {
+	cloned := []telegraf.Metric{}
+
+	for _, metric := range in {
+		cloned = append(cloned, metric.Copy())
+
+		if len(c.NameOverride) > 0 {
+			metric.SetName(c.NameOverride)
+		}
+		if len(c.NamePrefix) > 0 {
+			metric.AddPrefix(c.NamePrefix)
+		}
+		if len(c.NameSuffix) > 0 {
+			metric.AddSuffix(c.NameSuffix)
+		}
+		for key, value := range c.Tags {
+			metric.AddTag(key, value)
+		}
+	}
+	return append(in, cloned...)
+}
+
+func init() {
+	processors.Add("clone", func() telegraf.Processor {
+		return &Clone{}
+	})
+}

plugins/processors/clone/clone_test.go

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+package clone
+
+import (
+	"testing"
+	"time"
+
+	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/metric"
+	"github.com/stretchr/testify/assert"
+)
+
+func createTestMetric() telegraf.Metric {
+	metric, _ := metric.New("m1",
+		map[string]string{"metric_tag": "from_metric"},
+		map[string]interface{}{"value": int64(1)},
+		time.Now(),
+	)
+	return metric
+}
+
+func calculateProcessedTags(processor Clone, metric telegraf.Metric) map[string]string {
+	processed := processor.Apply(metric)
+	return processed[0].Tags()
+}
+
+func TestRetainsTags(t *testing.T) {
+	processor := Clone{}
+
+	tags := calculateProcessedTags(processor, createTestMetric())
+
+	value, present := tags["metric_tag"]
+	assert.True(t, present, "Tag of metric was not present")
+	assert.Equal(t, "from_metric", value, "Value of Tag was changed")
+}
+
+func TestAddTags(t *testing.T) {
+	processor := Clone{Tags: map[string]string{"added_tag": "from_config", "another_tag": ""}}
+
+	tags := calculateProcessedTags(processor, createTestMetric())
+
+	value, present := tags["added_tag"]
+	assert.True(t, present, "Additional Tag of metric was not present")
+	assert.Equal(t, "from_config", value, "Value of Tag was changed")
+	assert.Equal(t, 3, len(tags), "Should have one previous and two added tags.")
+}
+
+func TestOverwritesPresentTagValues(t *testing.T) {
+	processor := Clone{Tags: map[string]string{"metric_tag": "from_config"}}
+
+	tags := calculateProcessedTags(processor, createTestMetric())
+
+	value, present := tags["metric_tag"]
+	assert.True(t, present, "Tag of metric was not present")
+	assert.Equal(t, 1, len(tags), "Should only have one tag.")
+	assert.Equal(t, "from_config", value, "Value of Tag was not changed")
+}
+
+func TestOverridesName(t *testing.T) {
+	processor := Clone{NameOverride: "overridden"}
+
+	processed := processor.Apply(createTestMetric())
+
+	assert.Equal(t, "overridden", processed[0].Name(), "Name was not overridden")
+	assert.Equal(t, "m1", processed[1].Name(), "Original metric was modified")
+}
+
+func TestNamePrefix(t *testing.T) {
+	processor := Clone{NamePrefix: "Pre-"}
+
+	processed := processor.Apply(createTestMetric())
+
+	assert.Equal(t, "Pre-m1", processed[0].Name(), "Prefix was not applied")
+	assert.Equal(t, "m1", processed[1].Name(), "Original metric was modified")
+}
+
+func TestNameSuffix(t *testing.T) {
+	processor := Clone{NameSuffix: "-suff"}
+
+	processed := processor.Apply(createTestMetric())
+
+	assert.Equal(t, "m1-suff", processed[0].Name(), "Suffix was not applied")
+	assert.Equal(t, "m1", processed[1].Name(), "Original metric was modified")
+}