Add pivot and unpivot processors (#5991)

plugins/processors/unpivot/README.md

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+# Unpivot Processor
+
+You can use the `unpivot` processor to rotate a multi field series into single valued metrics.  This transformation often results in data that is more easy to aggregate across fields.
+
+To perform the reverse operation use the [pivot] processor.
+
+### Configuration
+
+```toml
+[[processors.unpivot]]
+  ## Tag to use for the name.
+  tag_key = "name"
+  ## Field to use for the name of the value.
+  value_key = "value"
+```
+
+### Example
+
+```diff
+- cpu,cpu=cpu0 time_idle=42i,time_user=43i
++ cpu,cpu=cpu0,name=time_idle value=42i
++ cpu,cpu=cpu0,name=time_user value=43i
+```
+
+[pivot]: /plugins/processors/pivot/README.md
+

plugins/processors/unpivot/unpivot.go

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+package unpivot
+
+import (
+	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/plugins/processors"
+)
+
+const (
+	description  = "Rotate multi field metric into several single field metrics"
+	sampleConfig = `
+  ## Tag to use for the name.
+  tag_key = "name"
+  ## Field to use for the name of the value.
+  value_key = "value"
+`
+)
+
+type Unpivot struct {
+	TagKey   string `toml:"tag_key"`
+	ValueKey string `toml:"value_key"`
+}
+
+func (p *Unpivot) SampleConfig() string {
+	return sampleConfig
+}
+
+func (p *Unpivot) Description() string {
+	return description
+}
+
+func copyWithoutFields(metric telegraf.Metric) telegraf.Metric {
+	m := metric.Copy()
+
+	fieldKeys := make([]string, 0, len(m.FieldList()))
+	for _, field := range m.FieldList() {
+		fieldKeys = append(fieldKeys, field.Key)
+	}
+
+	for _, fk := range fieldKeys {
+		m.RemoveField(fk)
+	}
+
+	return m
+}
+
+func (p *Unpivot) Apply(metrics ...telegraf.Metric) []telegraf.Metric {
+	fieldCount := 0
+	for _, m := range metrics {
+		fieldCount += len(m.FieldList())
+	}
+
+	results := make([]telegraf.Metric, 0, fieldCount)
+
+	for _, m := range metrics {
+		base := copyWithoutFields(m)
+		for _, field := range m.FieldList() {
+			newMetric := base.Copy()
+			newMetric.AddField(p.ValueKey, field.Value)
+			newMetric.AddTag(p.TagKey, field.Key)
+			results = append(results, newMetric)
+		}
+		m.Accept()
+	}
+	return results
+}
+
+func init() {
+	processors.Add("unpivot", func() telegraf.Processor {
+		return &Unpivot{}
+	})
+}

plugins/processors/unpivot/unpivot_test.go

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+package unpivot
+
+import (
+	"testing"
+	"time"
+
+	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/testutil"
+)
+
+func TestUnpivot(t *testing.T) {
+	now := time.Now()
+	tests := []struct {
+		name     string
+		unpivot  *Unpivot
+		metrics  []telegraf.Metric
+		expected []telegraf.Metric
+	}{
+		{
+			name: "simple",
+			unpivot: &Unpivot{
+				TagKey:   "name",
+				ValueKey: "value",
+			},
+			metrics: []telegraf.Metric{
+				testutil.MustMetric("cpu",
+					map[string]string{},
+					map[string]interface{}{
+						"idle_time": int64(42),
+					},
+					now,
+				),
+			},
+			expected: []telegraf.Metric{
+				testutil.MustMetric("cpu",
+					map[string]string{
+						"name": "idle_time",
+					},
+					map[string]interface{}{
+						"value": int64(42),
+					},
+					now,
+				),
+			},
+		},
+		{
+			name: "multi fields",
+			unpivot: &Unpivot{
+				TagKey:   "name",
+				ValueKey: "value",
+			},
+			metrics: []telegraf.Metric{
+				testutil.MustMetric("cpu",
+					map[string]string{},
+					map[string]interface{}{
+						"idle_time": int64(42),
+						"idle_user": int64(43),
+					},
+					now,
+				),
+			},
+			expected: []telegraf.Metric{
+				testutil.MustMetric("cpu",
+					map[string]string{
+						"name": "idle_time",
+					},
+					map[string]interface{}{
+						"value": int64(42),
+					},
+					now,
+				),
+				testutil.MustMetric("cpu",
+					map[string]string{
+						"name": "idle_user",
+					},
+					map[string]interface{}{
+						"value": int64(43),
+					},
+					now,
+				),
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			actual := tt.unpivot.Apply(tt.metrics...)
+			testutil.RequireMetricsEqual(t, tt.expected, actual, testutil.SortMetrics())
+		})
+	}
+}