Implement telegraf's own full metric type

main reasons behind this:
- make adding/removing tags cheap
- make adding/removing fields cheap
- make parsing cheaper
- make parse -> decorate -> write out bytes metric flow much faster

Refactor serializer to use byte buffer

plugins/serializers/registry.go

@@ -18,8 +18,10 @@ type SerializerOutput interface {
 // Serializer is an interface defining functions that a serializer plugin must
 // satisfy.
 type Serializer interface {
-	// Serialize takes a single telegraf metric and turns it into a string.
-	Serialize(metric telegraf.Metric) ([]string, error)
+	// Serialize takes a single telegraf metric and turns it into a byte buffer.
+	// separate metrics should be separated by a newline, and there should be
+	// a newline at the end of the buffer.
+	Serialize(metric telegraf.Metric) ([]byte, error)
 }
 
 // Config is a struct that covers the data types needed for all serializer types,