fix(core): Ensure metrics past MAX_METRIC_BUFFER_SIZE are not swallowed (#18212)

Looks like we swallowed the metric that triggers a flush when
MAX_METRIC_BUFFER_SIZE is surpassed.

Test demonstrating issue:
[f0737fa](f0737fa)
Fix:
[1a4e02a](1a4e02a)

Related logs pr: #18207

Author: andreiborza

packages/core/src/metrics/internal.ts

@@ -88,14 +88,16 @@ function setMetricAttribute(
  */
 export function _INTERNAL_captureSerializedMetric(client: Client, serializedMetric: SerializedMetric): void {
   const bufferMap = _getBufferMap();
-
   const metricBuffer = _INTERNAL_getMetricBuffer(client);
+
   if (metricBuffer === undefined) {
     bufferMap.set(client, [serializedMetric]);
   } else {
-    bufferMap.set(client, [...metricBuffer, serializedMetric]);
     if (metricBuffer.length >= MAX_METRIC_BUFFER_SIZE) {
       _INTERNAL_flushMetricsBuffer(client, metricBuffer);
+      bufferMap.set(client, [serializedMetric]);
+    } else {
+      bufferMap.set(client, [...metricBuffer, serializedMetric]);
     }
   }
 }

packages/core/test/lib/metrics/internal.test.ts

@@ -256,7 +256,10 @@ describe('_INTERNAL_captureMetric', () => {
     // Add one more to trigger flush
     _INTERNAL_captureMetric({ type: 'counter', name: 'trigger.flush', value: 999 }, { scope });
 
-    expect(_INTERNAL_getMetricBuffer(client)).toEqual([]);
+    // After flushing the 1000 metrics, the new metric starts a fresh buffer with 1 item
+    const buffer = _INTERNAL_getMetricBuffer(client);
+    expect(buffer).toHaveLength(1);
+    expect(buffer?.[0]?.name).toBe('trigger.flush');
   });
 
   it('does not flush metrics buffer when it is empty', () => {