add timestamp selector to JSON parser

allow user to specify timestamp field instead of being forced to use local time

docs/DATA_FORMATS_INPUT.md

@@ -106,6 +106,8 @@ The JSON data format supports specifying "tag keys". If specified, keys
 will be searched for in the root-level of the JSON blob. If the key(s) exist,
 they will be applied as tags to the Telegraf metrics.
 
+JSON data format can specify the timestamp by "timestamp_selector" and then parse it using "timestamp_formatter"; this could be useful when dealing with metrics not generated locally.
+
 For example, if you had this configuration:
 
 ```toml
@@ -127,6 +129,11 @@ For example, if you had this configuration:
     "my_tag_1",
     "my_tag_2"
   ]
+
+  timestamp_selector = "@timestamp"
+  ## for more information about timestamp formatter, please refer to:
+  ## https://golang.org/src/time/format.go
+  timestamp_formatter = "2006-01-02T15:04:05Z07:00"
 ```
 
 with this JSON output from a command:
@@ -137,14 +144,15 @@ with this JSON output from a command:
     "b": {
         "c": 6
     },
-    "my_tag_1": "foo"
+    "my_tag_1": "foo",
+    "@timestamp": "2016-07-27T16:46:00.554Z"
 }
 ```
 
-Your Telegraf metrics would get tagged with "my_tag_1"
+Your Telegraf metrics would get tagged with "my_tag_1" and timestamp
 
 ```
-exec_mycollector,my_tag_1=foo a=5,b_c=6
+exec_mycollector,my_tag_1=foo a=5,b_c=6 1469637960554000000
 ```
 
 # Value:

plugins/parsers/json/parser.go

@@ -14,6 +14,8 @@ type JSONParser struct {
 	MetricName         string
 	TagKeys            []string
 	DefaultTags        map[string]string
+	TimestampSelector  string
+	TimestampFormatter string
 }
 
 func (p *JSONParser) Parse(buf []byte) ([]telegraf.Metric, error) {
@@ -31,6 +33,25 @@ func (p *JSONParser) Parse(buf []byte) ([]telegraf.Metric, error) {
 		tags[k] = v
 	}
 
+	timestampStr := ""
+	timestampData := jsonOut[p.TimestampSelector]
+	if timestampData != nil {
+		timestampStr = timestampData.(string)
+	}
+
+	if p.TimestampFormatter == "" {
+		p.TimestampFormatter = time.RFC3339Nano
+	}
+
+	timestamp := time.Now().UTC()
+	if timestampStr != "" {
+		timestampTmp, err := time.Parse(p.TimestampFormatter, timestampStr)
+		if err != nil {
+			return nil, err
+		}
+		timestamp = timestampTmp
+	}
+
 	for _, tag := range p.TagKeys {
 		switch v := jsonOut[tag].(type) {
 		case string:
@@ -45,7 +66,7 @@ func (p *JSONParser) Parse(buf []byte) ([]telegraf.Metric, error) {
 		return nil, err
 	}
 
-	metric, err := telegraf.NewMetric(p.MetricName, tags, f.Fields, time.Now().UTC())
+	metric, err := telegraf.NewMetric(p.MetricName, tags, f.Fields, timestamp)
 
 	if err != nil {
 		return nil, err

plugins/parsers/registry.go

@@ -58,6 +58,12 @@ type Config struct {
 
 	// DefaultTags are the default tags that will be added to all parsed metrics.
 	DefaultTags map[string]string
+
+	// TimestampSelector only applies to JSON
+	TimestampSelector string
+
+	// TimestampFormatter only applies to JSON
+	TimestampFormatter string
 }
 
 // NewParser returns a Parser interface based on the given config.
@@ -67,7 +73,8 @@ func NewParser(config *Config) (Parser, error) {
 	switch config.DataFormat {
 	case "json":
 		parser, err = NewJSONParser(config.MetricName,
-			config.TagKeys, config.DefaultTags)
+			config.TagKeys, config.DefaultTags,
+			config.TimestampSelector, config.TimestampFormatter)
 	case "value":
 		parser, err = NewValueParser(config.MetricName,
 			config.DataType, config.DefaultTags)
@@ -88,11 +95,23 @@ func NewJSONParser(
 	metricName string,
 	tagKeys []string,
 	defaultTags map[string]string,
+	timestampParameters ...string,
 ) (Parser, error) {
+	timestampSelector, timestampFormatter := "", ""
+	switch len(timestampParameters) {
+	case 2:
+		timestampFormatter = timestampParameters[1]
+		fallthrough
+	case 1:
+		timestampSelector = timestampParameters[0]
+	}
+
 	parser := &json.JSONParser{
 		MetricName:         metricName,
 		TagKeys:            tagKeys,
 		DefaultTags:        defaultTags,
+		TimestampSelector:  timestampSelector,
+		TimestampFormatter: timestampFormatter,
 	}
 	return parser, nil
 }