refactor(metrics): use documented node-apis

Use `process.memoryUsage()` instead of `v8.getHeapStatistics()`

Author: SimonSimCity

lib/metrics/runtime.js

@@ -1,7 +1,5 @@
 'use strict'
 
-const v8 = require('v8')
-
 const eventLoopMonitor = require('monitor-event-loop-delay')
 
 const activeHandles = typeof process._getActiveHandles === 'function'
@@ -43,10 +41,10 @@ class RuntimeCollector {
     this.stats['nodejs.eventloop.delay.avg.ms'] = loopDelay
     this.loopMonitor.reset()
 
-    // Heap
-    const heap = v8.getHeapStatistics()
-    this.stats['nodejs.memory.heap.allocated.bytes'] = heap.total_heap_size
-    this.stats['nodejs.memory.heap.used.bytes'] = heap.used_heap_size
+    // Memory / Heap
+    const memoryUsage = process.memoryUsage()
+    this.stats['nodejs.memory.heap.allocated.bytes'] = memoryUsage.heapTotal
+    this.stats['nodejs.memory.heap.used.bytes'] = memoryUsage.heapUsed
 
     if (cb) process.nextTick(cb)
   }