Add influxdb plugin

This was primarily intended to consume InfluxDB-style expvars,
particularly InfluxDB's `/debug/vars` endpoint.

That endpoint follows a structure like

```json
{
  "httpd::8086": {
    "name": "httpd",
    "tags": {
      "bind": ":8086"
    },
    "values": {
      "pointsWrittenOK": 33756,
      "queryReq": 19,
      "queryRespBytes": 26973,
      "req": 428,
      "writeReq": 205,
      "writeReqBytes": 3939161
    }
  }
}
```

There are an arbitrary number of top-level keys in the JSON response at
the configured URLs, and this plugin will iterate through all of their
values looking for objects with keys "name", "tags", and "values"
indicating a metric to be consumed by telegraf.

Running this on current master of InfluxDB, I am able to record nearly
the same information that is normally stored in the `_internal`
database; the only measurement missing from `_internal` is `runtime`,
which is present under the "memstats" key but does not follow the format
and so is not consumed in this plugin.

```
$ influx -database=telegraf -execute 'SHOW FIELD KEYS FROM /influxdb/'

name: influxdb_influxdb_engine
----------------------------
fieldKey
blksWrite
blksWriteBytes
blksWriteBytesC
pointsWrite
pointsWriteDedupe

name: influxdb_influxdb_httpd
---------------------------
fieldKey
pingReq
pointsWrittenOK
queryReq
queryRespBytes
req
writeReq
writeReqBytes

name: influxdb_influxdb_shard
---------------------------
fieldKey
fieldsCreate
seriesCreate
writePointsOk
writeReq

name: influxdb_influxdb_subscriber
--------------------------------
fieldKey
pointsWritten

name: influxdb_influxdb_wal
-------------------------
fieldKey
autoFlush
flushDuration
idleFlush
memSize
metaFlush
pointsFlush
pointsWrite
pointsWriteReq
seriesFlush

name: influxdb_influxdb_write
---------------------------
fieldKey
pointReq
pointReqLocal
req
subWriteOk
writeOk
```

plugins/influxdb/influxdb.go

@@ -0,0 +1,150 @@
+package influxdb
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+	"strings"
+	"sync"
+
+	"github.com/influxdb/telegraf/plugins"
+)
+
+type InfluxDB struct {
+	Name string
+	URLs []string `toml:"urls"`
+}
+
+func (*InfluxDB) Description() string {
+	return "Read InfluxDB-formatted JSON metrics from one or more HTTP endpoints"
+}
+
+func (*InfluxDB) SampleConfig() string {
+	return `
+  # Reads InfluxDB-formatted JSON from given URLs.
+  # Works with InfluxDB debug endpoints out of the box, but other services can use this format too.
+  # See the influxdb plugin's README for more details.
+  [[plugins.influxdb]]
+  # Name to use for measurement
+  name = "influxdb"
+
+  # Multiple URLs from which to read InfluxDB-formatted JSON
+  urls = [
+    "http://localhost:8086/debug/vars"
+  ]
+`
+}
+
+func (i *InfluxDB) Gather(acc plugins.Accumulator) error {
+	errorChannel := make(chan error, len(i.URLs))
+
+	var wg sync.WaitGroup
+	for _, u := range i.URLs {
+		wg.Add(1)
+		go func(url string) {
+			defer wg.Done()
+			if err := i.gatherURL(acc, url); err != nil {
+				errorChannel <- fmt.Errorf("[name=%s][url=%s]: %s", i.Name, url, err)
+			}
+		}(u)
+	}
+
+	wg.Wait()
+	close(errorChannel)
+
+	// If there weren't any errors, we can return nil now.
+	if len(errorChannel) == 0 {
+		return nil
+	}
+
+	// There were errors, so join them all together as one big error.
+	errorStrings := make([]string, 0, len(errorChannel))
+	for err := range errorChannel {
+		errorStrings = append(errorStrings, err.Error())
+	}
+
+	return errors.New(strings.Join(errorStrings, "\n"))
+}
+
+type point struct {
+	Name   string                 `json:"name"`
+	Tags   map[string]string      `json:"tags"`
+	Values map[string]interface{} `json:"values"`
+}
+
+// Gathers data from a particular URL
+// Parameters:
+//     acc    : The telegraf Accumulator to use
+//     url    : endpoint to send request to
+//
+// Returns:
+//     error: Any error that may have occurred
+func (i *InfluxDB) gatherURL(
+	acc plugins.Accumulator,
+	url string,
+) error {
+	resp, err := http.Get(url)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	// It would be nice to be able to decode into a map[string]point, but
+	// we'll get a decoder error like:
+	// `json: cannot unmarshal array into Go value of type influxdb.point`
+	// if any of the values aren't objects.
+	// To avoid that error, we decode by hand.
+	dec := json.NewDecoder(resp.Body)
+
+	// Parse beginning of object
+	if t, err := dec.Token(); err != nil {
+		return err
+	} else if t != json.Delim('{') {
+		return errors.New("document root must be a JSON object")
+	}
+
+	// Loop through rest of object
+	for {
+		// Nothing left in this object, we're done
+		if !dec.More() {
+			break
+		}
+
+		// Read in a string key. We don't do anything with the top-level keys, so it's discarded.
+		_, err := dec.Token()
+		if err != nil {
+			return err
+		}
+
+		// Attempt to parse a whole object into a point.
+		// It might be a non-object, like a string or array.
+		// If we fail to decode it into a point, ignore it and move on.
+		var p point
+		if err := dec.Decode(&p); err != nil {
+			continue
+		}
+
+		// If the object was a point, but was not fully initialized, ignore it and move on.
+		if p.Name == "" || p.Tags == nil || p.Values == nil || len(p.Values) == 0 {
+			continue
+		}
+
+		// Add a tag to indicate the source of the data.
+		p.Tags["url"] = url
+
+		acc.AddFields(
+			i.Name+"_"+p.Name,
+			p.Values,
+			p.Tags,
+		)
+	}
+
+	return nil
+}
+
+func init() {
+	plugins.Add("influxdb", func() plugins.Plugin {
+		return &InfluxDB{}
+	})
+}