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/json/parser.go

@@ -0,0 +1,109 @@
+package json
+
+import (
+	"encoding/json"
+	"fmt"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/influxdata/telegraf"
+)
+
+type JSONParser struct {
+	MetricName  string
+	TagKeys     []string
+	DefaultTags map[string]string
+}
+
+func (p *JSONParser) Parse(buf []byte) ([]telegraf.Metric, error) {
+	metrics := make([]telegraf.Metric, 0)
+
+	var jsonOut map[string]interface{}
+	err := json.Unmarshal(buf, &jsonOut)
+	if err != nil {
+		err = fmt.Errorf("unable to parse out as JSON, %s", err)
+		return nil, err
+	}
+
+	tags := make(map[string]string)
+	for k, v := range p.DefaultTags {
+		tags[k] = v
+	}
+
+	for _, tag := range p.TagKeys {
+		switch v := jsonOut[tag].(type) {
+		case string:
+			tags[tag] = v
+		}
+		delete(jsonOut, tag)
+	}
+
+	f := JSONFlattener{}
+	err = f.FlattenJSON("", jsonOut)
+	if err != nil {
+		return nil, err
+	}
+
+	metric, err := telegraf.NewMetric(p.MetricName, tags, f.Fields, time.Now().UTC())
+
+	if err != nil {
+		return nil, err
+	}
+	return append(metrics, metric), nil
+}
+
+func (p *JSONParser) ParseLine(line string) (telegraf.Metric, error) {
+	metrics, err := p.Parse([]byte(line + "\n"))
+
+	if err != nil {
+		return nil, err
+	}
+
+	if len(metrics) < 1 {
+		return nil, fmt.Errorf("Can not parse the line: %s, for data format: influx ", line)
+	}
+
+	return metrics[0], nil
+}
+
+type JSONFlattener struct {
+	Fields map[string]interface{}
+}
+
+// FlattenJSON flattens nested maps/interfaces into a fields map
+func (f *JSONFlattener) FlattenJSON(
+	fieldname string,
+	v interface{},
+) error {
+	if f.Fields == nil {
+		f.Fields = make(map[string]interface{})
+	}
+	fieldname = strings.Trim(fieldname, "_")
+	switch t := v.(type) {
+	case map[string]interface{}:
+		for k, v := range t {
+			err := f.FlattenJSON(fieldname+"_"+k+"_", v)
+			if err != nil {
+				return err
+			}
+		}
+	case []interface{}:
+		for i, v := range t {
+			k := strconv.Itoa(i)
+			err := f.FlattenJSON(fieldname+"_"+k+"_", v)
+			if err != nil {
+				return nil
+			}
+		}
+	case float64:
+		f.Fields[fieldname] = t
+	case bool, string, nil:
+		// ignored types
+		return nil
+	default:
+		return fmt.Errorf("JSON Flattener: got unexpected type %T with value %v (%s)",
+			t, t, fieldname)
+	}
+	return nil
+}