feat: declare collection cadence and preserve Agent sampling timestamps

tests/test_metrics.py

@@ -11,7 +11,7 @@ class MetricsTests(unittest.TestCase):
         value = {'timestamp': 123, 'machine': {'name':'Pi'}, 'system': {
             'cpu_percent': 10, 'memory_total_bytes': 8000, 'memory_used_bytes': 4000,
             'disk_total_bytes': 16000, 'disk_used_bytes': 5000},
-            'uptime_seconds': 1000, 'network': {'link_speed_mbps':1000,'rx_bytes_per_second':0,'tx_bytes_per_second':1234},
+            'uptime_seconds': 1000, 'network': {'observed_at':120,'link_speed_mbps':1000,'rx_bytes_per_second':0,'tx_bytes_per_second':1234},
             'display': {'alerts': []}}
         with patch.object(worker,'request',return_value=value): result=worker.collect({})
         values = {m['key']:m['value'] for m in result['metrics']}
@@ -20,6 +20,8 @@ class MetricsTests(unittest.TestCase):
         self.assertEqual(values['network_link_speed_mbps'],1000)
         self.assertEqual(values['network_rx_bytes_per_second'],0)
         self.assertEqual(values['network_tx_bytes_per_second'],1234)
+        self.assertEqual(next(m['observed_at'] for m in result['metrics'] if m['key']=='network_tx_bytes_per_second'),120)
+        self.assertEqual(result['collection']['interval_seconds'],1)
         self.assertTrue(result['alerts_complete'])
 
     def test_absent_or_boolean_fields_not_invented(self):