feat(metrics): add metrics for external memory

Added metrics for `nodejs.memory.external.bytes` and `nodejs.memory.arrayBuffers.bytes`

Author: SimonSimCity

docs/metrics.asciidoc

@@ -129,6 +129,25 @@ The current allocated heap size in bytes.
 
 The currently used heap size in bytes.
 
+[float]
+[[metric-nodejs.memory.external.bytes]]
+=== `nodejs.memory.external.bytes`
+
+* *Type:* Long
+* *Format:* Bytes
+
+Memory usage of C++ objects bound to JavaScript objects managed by V8.
+
+[float]
+[[metric-nodejs.memory.arrayBuffers.bytes]]
+=== `nodejs.memory.arrayBuffers.bytes`
+
+* *Type:* Long
+* *Format:* Bytes
+
+Memory allocated for ArrayBuffers and SharedArrayBuffers, including all Node.js Buffers.
+This is also included in the `nodejs.memory.external.bytes` value.
+
 [float]
 [[metrics-transaction.duration.sum]]
 === `transaction.duration.sum`

lib/metrics/runtime.js

@@ -19,7 +19,9 @@ class RuntimeCollector {
       'nodejs.requests.active': 0,
       'nodejs.eventloop.delay.ns': 0,
       'nodejs.memory.heap.allocated.bytes': 0,
-      'nodejs.memory.heap.used.bytes': 0
+      'nodejs.memory.heap.used.bytes': 0,
+      'nodejs.memory.external.bytes': 0,
+      'nodejs.memory.arrayBuffers.bytes': 0
     }
 
     const monitor = eventLoopMonitor({
@@ -46,6 +48,9 @@ class RuntimeCollector {
     this.stats['nodejs.memory.heap.allocated.bytes'] = memoryUsage.heapTotal
     this.stats['nodejs.memory.heap.used.bytes'] = memoryUsage.heapUsed
 
+    this.stats['nodejs.memory.external.bytes'] = memoryUsage.external
+    this.stats['nodejs.memory.arrayBuffers.bytes'] = memoryUsage.arrayBuffers || 0 // Only available in NodeJS +13.0
+
     if (cb) process.nextTick(cb)
   }
 }

test/metrics/index.js

@@ -117,6 +117,13 @@ test('reports expected metrics', function (t) {
       'nodejs.memory.heap.used.bytes': (value) => {
         t.ok(value >= 0, 'is positive')
       },
+      'nodejs.memory.external.bytes': (value) => {
+        const external = process.memoryUsage().external
+        t.ok(isRoughly(value, external, 0.1), `is close to current external (value: ${value}, external: ${external})`)
+      },
+      'nodejs.memory.arrayBuffers.bytes': (value) => {
+        t.ok(value >= 0, 'is positive')
+      },
       'ws.connections': (value) => {
         t.equal(value, 23)
       }