Add Factorio CPU and Memory usage statistics

Add per instance metrics for CPU and memory usage with the pidusage
library.

lib/factorio/server.js

@@ -590,6 +590,16 @@ class FactorioServer extends events.EventEmitter {
 		return this._version;
 	}
 
+	/**
+	 * The pid of the server process, or null if not running
+	 */
+	get pid() {
+		if (!this._server) {
+			return null;
+		}
+		return this._server.pid;
+	}
+
 	_attachStdio() {
 		this._server.stdout.on('data', chunk => { this._stdout.data(chunk); });
 		this._server.stdout.on('close', () => { this._stdout.end(); });