streaming processors docs update (influxdata#7786)

docs/PROCESSORS.md

@@ -64,6 +64,80 @@ func init() {
 }
 ```
 
+### Streaming Processors
+
+Streaming processors are a new processor type available to you. They are
+particularly useful to implement processor types that use background processes
+or goroutines to process multiple metrics at the same time. Some examples of this
+are the execd processor, which pipes metrics out to an external process over stdin
+and reads them back over stdout, and the reverse_dns processor, which does reverse
+dns lookups on IP addresses in fields. While both of these come with a speed cost,
+it would be significantly worse if you had to process one metric completely from
+start to finish before handling the next metric, and thus they benefit
+significantly from a streaming-pipe approach.
+
+Some differences from classic Processors:
+
+* Streaming processors must conform to the [telegraf.StreamingProcessor][] interface.
+* Processors should call `processors.AddStreaming` in their `init` function to register
+  themselves.  See below for a quick example.
+
+### Streaming Processor Example
+
+```go
+package printer
+
+// printer.go
+
+import (
+	"fmt"
+
+	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/plugins/processors"
+)
+
+type Printer struct {
+}
+
+var sampleConfig = `
+`
+
+func (p *Printer) SampleConfig() string {
+	return sampleConfig
+}
+
+func (p *Printer) Description() string {
+	return "Print all metrics that pass through this filter."
+}
+
+func (p *Printer) Init() error {
+	return nil
+}
+
+func (p *Printer) Start(acc telegraf.Accumulator) error {
+}
+
+func (p *Printer) Add(metric telegraf.Metric, acc telegraf.Accumulator) error {
+	// print!
+	fmt.Println(metric.String())
+	// pass the metric downstream, or metric.Drop() it.
+	// Metric will be dropped if this function returns an error.
+	acc.AddMetric(metric)
+
+	return nil
+}
+
+func (p *Printer) Stop() error {
+}
+
+func init() {
+	processors.AddStreaming("printer", func() telegraf.StreamingProcessor {
+		return &Printer{}
+	})
+}
+```
+
 [SampleConfig]: https://github.com/influxdata/telegraf/wiki/SampleConfig
 [CodeStyle]: https://github.com/influxdata/telegraf/wiki/CodeStyle
 [telegraf.Processor]: https://godoc.org/github.com/influxdata/telegraf#Processor
+[telegraf.StreamingProcessor]: https://godoc.org/github.com/influxdata/telegraf#StreamingProcessor