lua: Implement "Hello world" USDT example

examples/lua/usdt_ruby.lua

+#!/usr/bin/env bcc-lua
+--[[
+Copyright 2016 GitHub, Inc
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+]]
+
+local program = [[
+#include <uapi/linux/ptrace.h>
+int trace_method(struct pt_regs *ctx) {
+    char fn_name[128] = {};
+    bpf_usdt_readarg_p(method__entry_2, ctx, &fn_name, sizeof(fn_name));
+    bpf_trace_printk("%s(...)\n", &fn_name);
+    return 0;
+};
+]]
+
+return function(BPF, util)
+  if not arg[1] then
+    print("usage: rubysyms.lua PID")
+    return
+  end
+
+  local u = util.USDT:new{pid=tonumber(arg[1])}
+  u:enable_probe{probe="method__entry", fn_name="trace_method"}
+
+  local b = BPF:new{text=program, usdt=u}
+  local pipe = b:pipe()
+  while true do
+    print(pipe:trace_fields())
+  end
+end