add an admin-rpc to count the number of open files

lib/commands/admin-rpc.js

@@ -7,6 +7,13 @@ var Fs = require("fs");
 
 var Admin = module.exports;
 
+var getFileDescriptorCount = function (Env, server, cb) {
+    Fs.readdir('/proc/self/fd', function(err, list) {
+        if (err) { return void cb(err); }
+        cb(void 0, list.length);
+    });
+};
+
 var getActiveSessions = function (Env, Server, cb) {
     var stats = Server.getSessionStats();
     cb(void 0, [
@@ -105,6 +112,7 @@ var commands = {
     DISK_USAGE: getDiskUsage,
     FLUSH_CACHE: flushCache,
     SHUTDOWN: shutdown,
+    GET_FILE_DESCRIPTOR_COUNT: getFileDescriptorCount,
 };
 
 Admin.command = function (Env, safeKey, data, _cb, Server) {