Fix typo in default cpu metric

Fix proces_cpu_seconds_total being mislabled as process_cpu_seconds.

lib/prometheus.js

@@ -400,7 +400,8 @@ function deserializeResult(serializedResult) {
 
 let defaultCollectors = {};
 defaultCollectors.processCpuSecondsTotal = new Gauge(
-	'process_cpu_seconds', "Total user and system CPU time spent in seconds.",
+	'process_cpu_seconds_total',
+	"Total user and system CPU time spent in seconds.",
 	{
 		callback: async function() {
 			let usage = process.cpuUsage();