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_test.go

@@ -0,0 +1,101 @@
+package influxdb_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/influxdb/telegraf/plugins/influxdb"
+	"github.com/influxdb/telegraf/testutil"
+	"github.com/stretchr/testify/require"
+)
+
+func TestBasic(t *testing.T) {
+	js := `
+{
+  "_1": {
+    "name": "foo",
+    "tags": {
+      "id": "ex1"
+    },
+    "values": {
+      "i": -1,
+      "f": 0.5,
+      "b": true,
+      "s": "string"
+    }
+  },
+  "ignored": {
+    "willBeRecorded": false
+  },
+  "ignoredAndNested": {
+    "hash": {
+      "is": "nested"
+    }
+  },
+  "array": [
+   "makes parsing more difficult than necessary"
+  ],
+  "string": "makes parsing more difficult than necessary",
+  "_2": {
+    "name": "bar",
+    "tags": {
+      "id": "ex2"
+    },
+    "values": {
+      "x": "x"
+    }
+  },
+  "pointWithoutFields_willNotBeIncluded": {
+    "name": "asdf",
+    "tags": {
+      "id": "ex3"
+    },
+    "values": {}
+  }
+}
+`
+	fakeServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path == "/endpoint" {
+			_, _ = w.Write([]byte(js))
+		} else {
+			w.WriteHeader(http.StatusNotFound)
+		}
+	}))
+	defer fakeServer.Close()
+
+	plugin := &influxdb.InfluxDB{
+		Name: "test",
+		URLs: []string{fakeServer.URL + "/endpoint"},
+	}
+
+	var acc testutil.Accumulator
+	require.NoError(t, plugin.Gather(&acc))
+
+	require.Len(t, acc.Points, 2)
+	require.NoError(t, acc.ValidateTaggedFieldsValue(
+		"test_foo",
+		map[string]interface{}{
+			// JSON will truncate floats to integer representations.
+			// Since there's no distinction in JSON, we can't assume it's an int.
+			"i": -1.0,
+			"f": 0.5,
+			"b": true,
+			"s": "string",
+		},
+		map[string]string{
+			"id":  "ex1",
+			"url": fakeServer.URL + "/endpoint",
+		},
+	))
+	require.NoError(t, acc.ValidateTaggedFieldsValue(
+		"test_bar",
+		map[string]interface{}{
+			"x": "x",
+		},
+		map[string]string{
+			"id":  "ex2",
+			"url": fakeServer.URL + "/endpoint",
+		},
+	))
+}