perf: use the faster api to collect rss

Author: chidam333

lib/metrics/helpers/safeRss.js

@@ -0,0 +1,12 @@
+'use strict';
+
+// process.memoryUsage.rss() can throw on some platforms, see #67
+function safeRss() {
+	try {
+		return process.memoryUsage.rss();
+	} catch {
+		return;
+	}
+}
+
+module.exports = safeRss;

lib/metrics/osMemoryHeap.js

@@ -2,7 +2,7 @@
 
 const Gauge = require('../gauge');
 const linuxVariant = require('./osMemoryHeapLinux');
-const safeMemoryUsage = require('./helpers/safeMemoryUsage');
+const safeRss = require('./helpers/safeRss');
 
 const PROCESS_RESIDENT_MEMORY = 'process_resident_memory_bytes';
 
@@ -17,11 +17,11 @@ function notLinuxVariant(registry, config = {}) {
 		registers: registry ? [registry] : undefined,
 		labelNames,
 		collect() {
-			const memUsage = safeMemoryUsage();
-
-			// I don't think the other things returned from `process.memoryUsage()` is relevant to a standard export
-			if (memUsage) {
-				this.set(labels, memUsage.rss);
+			// process.memoryUsage.rss() is faster than process.memoryUsage() as it
+			// only reads RSS from the OS without collecting all heap statistics
+			const rss = safeRss();
+			if (rss !== undefined) {
+				this.set(labels, rss);
 			}
 		},
 	});