refactor: use async fs/promises in osMemoryHeapLinux

Changed osMemoryHeapLinux to use async readFile from fs/promises
instead of synchronous readFileSync for better non-blocking I/O.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: 2 people

lib/metrics/osMemoryHeapLinux.js

@@ -1,7 +1,7 @@
 'use strict';
 
 const Gauge = require('../gauge');
-const fs = require('fs');
+const { readFile } = require('fs/promises');
 
 const values = ['VmSize', 'VmRSS', 'VmData'];
 
@@ -42,15 +42,14 @@ module.exports = (registry, config = {}) => {
 		registers,
 		labelNames,
 		// Use this one metric's `collect` to set all metrics' values.
-		collect() {
+		async collect() {
 			try {
-				// Sync I/O is often problematic, but /proc isn't really I/O, it
-				// a virtual filesystem that maps directly to in-kernel data
+				// /proc is a virtual filesystem that maps directly to in-kernel data
 				// structures and never blocks.
 				//
 				// Node.js/libuv do this already for process.memoryUsage(), see:
 				// - https://github.com/libuv/libuv/blob/a629688008694ed8022269e66826d4d6ec688b83/src/unix/linux-core.c#L506-L523
-				const stat = fs.readFileSync('/proc/self/status', 'utf8');
+				const stat = await readFile('/proc/self/status', 'utf8');
 				const structuredOutput = structureOutput(stat);
 
 				residentMemGauge.set(labels, structuredOutput.VmRSS);