Add call to optional Init function for all plugins (#5899)

docs/AGGREGATORS.md

@@ -52,6 +52,10 @@ var sampleConfig = `
   drop_original = false
 `
 
+func (m *Min) Init() error {
+	return nil
+}
+
 func (m *Min) SampleConfig() string {
 	return sampleConfig
 }

docs/INPUTS.md

@@ -52,6 +52,10 @@ func (s *Simple) SampleConfig() string {
 `
 }
 
+func (s *Simple) Init() error {
+	return nil
+}
+
 func (s *Simple) Gather(acc telegraf.Accumulator) error {
     if s.Ok {
         acc.AddFields("state", map[string]interface{}{"value": "pretty good"}, nil)

docs/OUTPUTS.md

@@ -43,6 +43,10 @@ func (s *Simple) SampleConfig() string {
 `
 }
 
+func (s *Simple) Init() error {
+	return nil
+}
+
 func (s *Simple) Connect() error {
     // Make a connection to the URL here
     return nil

docs/PROCESSORS.md

@@ -46,6 +46,10 @@ func (p *Printer) Description() string {
 	return "Print all metrics that pass through this filter."
 }
 
+func (p *Printer) Init() error {
+	return nil
+}
+
 func (p *Printer) Apply(in ...telegraf.Metric) []telegraf.Metric {
 	for _, metric := range in {
 		fmt.Println(metric.String())