Register resident set size and limit bytes metrics in APM (#215458)

## Summary

Resolves elastic/kibana-team#1565

Register resident set size and limit bytes metrics in APM.

<img width="1614" alt="image"
src="https://github.com/user-attachments/assets/d5f1caf5-da4f-4acd-a63f-2b5256a91307"
/>


### Checklist

Check the PR satisfies following conditions. 

Reviewers should verify this PR satisfies this list as well.

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

Author: jesuswr

src/core/packages/metrics/collectors-server-internal/src/process.test.ts

@@ -10,6 +10,7 @@
 import v8, { HeapInfo } from 'v8';
 import { mockEventLoopDelayMonitor, mockEventLoopUtilizationMonitor } from './process.test.mocks';
 import { ProcessMetricsCollector } from './process';
+import apm from 'elastic-apm-node';
 
 describe('ProcessMetricsCollector', () => {
   let collector: ProcessMetricsCollector;
@@ -103,4 +104,24 @@ describe('ProcessMetricsCollector', () => {
       expect(mockEventLoopUtilizationMonitor.reset).toBeCalledTimes(1);
     });
   });
+
+  describe('register metrics in apm', () => {
+    it('calls registerMetric in the constructor', () => {
+      const apmSpy = jest.spyOn(apm, 'registerMetric');
+
+      collector.registerMetrics();
+
+      expect(apmSpy).toHaveBeenCalledTimes(2);
+      expect(apmSpy).toHaveBeenNthCalledWith(
+        1,
+        'nodejs.memory.resident_set_size.bytes',
+        expect.any(Function)
+      );
+      expect(apmSpy).toHaveBeenNthCalledWith(
+        2,
+        'nodejs.heap.size_limit.bytes',
+        expect.any(Function)
+      );
+    });
+  });
 });

src/core/packages/metrics/collectors-server-internal/src/process.ts

@@ -9,6 +9,7 @@
 
 import v8 from 'v8';
 import type { OpsProcessMetrics, MetricsCollector } from '@kbn/core-metrics-server';
+import apm from 'elastic-apm-node';
 import { EventLoopDelaysMonitor } from './event_loop_delays_monitor';
 import { EventLoopUtilizationMonitor } from './event_loop_utilization_monitor';
 
@@ -54,6 +55,14 @@ export class ProcessMetricsCollector implements MetricsCollector<OpsProcessMetri
     return [this.getCurrentPidMetrics()];
   }
 
+  public registerMetrics() {
+    apm.registerMetric('nodejs.memory.resident_set_size.bytes', () => process.memoryUsage().rss);
+    apm.registerMetric(
+      'nodejs.heap.size_limit.bytes',
+      () => v8.getHeapStatistics().heap_size_limit
+    );
+  }
+
   public reset() {
     this.eventLoopDelayMonitor.reset();
     this.eventLoopUtilizationMonitor.reset();

src/core/packages/metrics/collectors-server-mocks/src/mocks.ts

@@ -21,7 +21,13 @@ const createMock = () => {
   return mocked;
 };
 
+const createMockProcessMetricsCollector = () => ({
+  ...createMock(),
+  registerMetrics: jest.fn().mockResolvedValue({}),
+});
+
 export const collectorMock = {
   create: createMock,
   createOpsProcessMetrics: createMockOpsProcessMetrics,
+  createProcessMetricsCollector: createMockProcessMetricsCollector,
 };

src/core/packages/metrics/server-internal/src/metrics_service.test.mocks.ts

@@ -9,7 +9,10 @@
 
 import { collectorMock } from '@kbn/core-metrics-collectors-server-mocks';
 
-export const mockOpsCollector = collectorMock.create();
+export const mockOpsCollector = {
+  ...collectorMock.create(),
+  registerMetrics: jest.fn(),
+};
 
 jest.doMock('./ops_metrics_collector', () => ({
   OpsMetricsCollector: jest.fn().mockImplementation(() => mockOpsCollector),

src/core/packages/metrics/server-internal/src/metrics_service.ts

@@ -78,6 +78,8 @@ export class MetricsService
       }
     );
 
+    this.metricsCollector.registerMetrics();
+
     await this.refreshMetrics();
 
     this.collectInterval = setInterval(() => {

src/core/packages/metrics/server-internal/src/ops_metrics_collector.test.mocks.ts

@@ -10,7 +10,7 @@
 import { collectorMock } from '@kbn/core-metrics-collectors-server-mocks';
 
 export const mockOsCollector = collectorMock.create();
-export const mockProcessCollector = collectorMock.create();
+export const mockProcessCollector = collectorMock.createProcessMetricsCollector();
 export const mockServerCollector = collectorMock.create();
 export const mockEsClientCollector = collectorMock.create();
 

src/core/packages/metrics/server-internal/src/ops_metrics_collector.test.ts

@@ -80,4 +80,12 @@ describe('OpsMetricsCollector', () => {
       expect(mockServerCollector.reset).toHaveBeenCalledTimes(2);
     });
   });
+
+  describe('#registerMetrics', () => {
+    it('call registerMetrics on the underlying collectors', () => {
+      collector.registerMetrics();
+
+      expect(mockProcessCollector.registerMetrics).toHaveBeenCalledTimes(1);
+    });
+  });
 });

src/core/packages/metrics/server-internal/src/ops_metrics_collector.ts

@@ -64,6 +64,10 @@ export class OpsMetricsCollector implements MetricsCollector<OpsMetrics> {
     };
   }
 
+  public registerMetrics() {
+    this.processCollector.registerMetrics();
+  }
+
   public reset() {
     this.processCollector.reset();
     this.osCollector.reset();