FIX: Fork collector demon in a safer way. (#126)

Previously we were using the following pattern to fork a child process

```
if @pid = fork
  write_pid_file
  return
end

<Do something in child process>
```

However, we noticed that if any error occurs in the child process (ie.
after the `if @pid = fork` line), the error propogates to the parent
process as well causing it to crash. Instead, we can pass `fork` a block
instead and that does not cause the error to propogate to the parent
process.

Author: tgxworld

lib/collector_demon.rb

@@ -8,21 +8,23 @@ def self.prefix
   end
 
   def run
-    if @pid = fork
-      write_pid_file
-      return
-    end
+    @pid =
+      fork do
+        collector = File.expand_path("../../bin/collector", __FILE__)
 
-    collector = File.expand_path("../../bin/collector", __FILE__)
+        ENV["RUBY_GLOBAL_METHOD_CACHE_SIZE"] = "2048"
+        ENV["RUBY_GC_HEAP_INIT_SLOTS"] = "10000"
+        ENV["PROMETHEUS_EXPORTER_VERSION"] = PrometheusExporter::VERSION
 
-    ENV["RUBY_GLOBAL_METHOD_CACHE_SIZE"] = "2048"
-    ENV["RUBY_GC_HEAP_INIT_SLOTS"] = "10000"
-    ENV["PROMETHEUS_EXPORTER_VERSION"] = PrometheusExporter::VERSION
+        exec collector,
+             GlobalSetting.prometheus_collector_port.to_s,
+             GlobalSetting.prometheus_webserver_bind,
+             parent_pid.to_s,
+             pid_file
+      end
 
-    exec collector,
-         GlobalSetting.prometheus_collector_port.to_s,
-         GlobalSetting.prometheus_webserver_bind,
-         parent_pid.to_s,
-         pid_file
+    Process.detach(@pid)
+
+    write_pid_file
   end
 end