Added support for using default route

This is useful for dynamically setting the host in containers.
For example, most Kubernetes nodes will have a statsd server on them,
and using this will allow containers to send metrics directly to that
without extra configuration.

(Inspired by the same option in the datadogpy library)

Tested by:
    Checking the output of the getDefaultRoute() function

README.md

@@ -36,6 +36,7 @@ Parameters (specified as an options hash):
 * `telegraf`:    Use Telegraf's StatsD line protocol, which is slightly different than the rest `default: false`
 * `sampleRate`:    Sends only a sample of data to StatsD for all StatsD methods.  Can be overriden at the method level. `default: 1`
 * `errorHandler`: A function with one argument. It is called to handle various errors. `default: none`, errors are thrown/logger to console
+* `useDefaultRoute`: Use the default interface on a Linux system. Useful when running in containers
 
 All StatsD methods other than event and close have the same API:
 * `name`:       Stat name `required`

lib/statsd.js

@@ -2,7 +2,8 @@
 
 var dgram = require('dgram'),
     util = require('util'),
-    dns   = require('dns');
+    dns   = require('dns'),
+    fs    = require('fs');
 
 /**
  * The UDP Client for StatsD
@@ -19,6 +20,7 @@ var dgram = require('dgram'),
  *   @maxBufferSize      {Number} An optional value for aggregating metrics to send, mainly for performance improvement
  *   @bufferFlushInterval {Number} the time out value to flush out buffer if not
  *   @option sampleRate {Float} Global Sampling rate, default: 1 (No sampling)
+ *   @option useDefaultRoute {boolean} An optional boolean to use the default route on Linux. Useful for containers
  * @constructor
  */
 var Client = function (host, port, prefix, suffix, globalize, cacheDns, mock,
@@ -87,6 +89,14 @@ var Client = function (host, port, prefix, suffix, globalize, cacheDns, mock,
     global.statsd = this;
   }
 
+  if(options.useDefaultRoute) {
+    var defaultRoute = getDefaultRoute();
+    if (defaultRoute) {
+      console.log('Got ' + defaultRoute + " for the system's default route");
+      this.host = defaultRoute;
+    }
+  }
+
   this.CHECKS = {
     OK: 0,
     WARNING: 1,
@@ -690,5 +700,35 @@ function formatDate(date) {
   return timestamp;
 }
 
+function intToIP(int) {
+  var part1 = int & 255;
+  var part2 = ((int >> 8) & 255);
+  var part3 = ((int >> 16) & 255);
+  var part4 = ((int >> 24) & 255);
+
+  return part4 + "." + part3 + "." + part2 + "." + part1;
+}
+
+// Returns the system default interface
+function getDefaultRoute() {
+  try {
+    var fileContents = fs.readFileSync('/proc/net/route');
+    var routes = fileContents.split('\n');
+    for (var route in routes) {
+      var fields = route.trim().split('');
+      if (fields[1] === '00000000') {
+        var address = fields[2];
+        // Convert to little endian by splitting every 2 digits and reversing that list
+        var littleEndianAddress = address.match(/.{2}/g).reverse().join("");
+        return intToIP(parseInt(littleEndianAddress, 16));
+      }
+    }
+  } catch (e) {
+    console.error('Could not get default route from /proc/net/route');
+  }
+  return null;
+}
+
+
 exports = module.exports = Client;
 exports.StatsD = Client;

types.d.ts

@@ -19,6 +19,7 @@ declare module "hot-shots" {
     socket?: dgram.Socket;
     suffix?: string;
     telegraf?: boolean;
+    useDefaultRoute?: boolean;
   }
 
   export interface ChildClientOptions {