Implementing generic parser plugins and documentation

This constitutes a large change in how we will parse different data
formats going forward (for the plugins that support it)

This is working off @henrypfhu's changes.

plugins/parsers/registry.go

@@ -0,0 +1,95 @@
+package parsers
+
+import (
+	"fmt"
+
+	"github.com/influxdata/telegraf"
+
+	"github.com/influxdata/telegraf/plugins/parsers/graphite"
+	"github.com/influxdata/telegraf/plugins/parsers/influx"
+	"github.com/influxdata/telegraf/plugins/parsers/json"
+)
+
+// ParserInput is an interface for input plugins that are able to parse
+// arbitrary data formats.
+type ParserInput interface {
+	// SetParser sets the parser function for the interface
+	SetParser(parser Parser)
+}
+
+// Parser is an interface defining functions that a parser plugin must satisfy.
+type Parser interface {
+	// Parse takes a byte buffer separated by newlines
+	// ie, `cpu.usage.idle 90\ncpu.usage.busy 10`
+	// and parses it into telegraf metrics
+	Parse(buf []byte) ([]telegraf.Metric, error)
+
+	// ParseLine takes a single string metric
+	// ie, "cpu.usage.idle 90"
+	// and parses it into a telegraf metric.
+	ParseLine(line string) (telegraf.Metric, error)
+}
+
+// Config is a struct that covers the data types needed for all parser types,
+// and can be used to instantiate _any_ of the parsers.
+type Config struct {
+	// Dataformat can be one of: json, influx, graphite
+	DataFormat string
+
+	// Separator only applied to Graphite data.
+	Separator string
+	// Templates only apply to Graphite data.
+	Templates []string
+
+	// TagKeys only apply to JSON data
+	TagKeys []string
+	// MetricName only applies to JSON data. This will be the name of the measurement.
+	MetricName string
+
+	// DefaultTags are the default tags that will be added to all parsed metrics.
+	DefaultTags map[string]string
+}
+
+// NewParser returns a Parser interface based on the given config.
+func NewParser(config *Config) (Parser, error) {
+	var err error
+	var parser Parser
+	switch config.DataFormat {
+	case "json":
+		parser, err = NewJSONParser(config.MetricName,
+			config.TagKeys, config.DefaultTags)
+	case "influx":
+		parser, err = NewInfluxParser()
+	case "graphite":
+		parser, err = NewGraphiteParser(config.Separator,
+			config.Templates, config.DefaultTags)
+	default:
+		err = fmt.Errorf("Invalid data format: %s", config.DataFormat)
+	}
+	return parser, err
+}
+
+func NewJSONParser(
+	metricName string,
+	tagKeys []string,
+	defaultTags map[string]string,
+) (Parser, error) {
+	parser := &json.JSONParser{
+		MetricName:  metricName,
+		TagKeys:     tagKeys,
+		DefaultTags: defaultTags,
+	}
+	return parser, nil
+}
+
+func NewInfluxParser() (Parser, error) {
+	return &influx.InfluxParser{}, nil
+}
+
+func NewGraphiteParser(
+	separator string,
+	templates []string,
+	defaultTags map[string]string,
+) (Parser, error) {
+	return graphite.NewGraphiteParser(separator, templates, defaultTags)
+}