fix metrics timestamp using monotonic clock

The utc_timestamp_ms field that is reported by the metrics
uses a CLOCK_MONOTONIC timestamp. The POSIX standard defines
the monotonic clock as a clock with a fixed but undefined
starting point.
For linux, the starting point is usually set at boot.

This commit replaces the clock with a CLOCK_REALTIME, which
gives the interval from the UTC UNIX epoch. It also updates
the changelog and adds a regression test.

Signed-off-by: alindima <alindima@amazon.com>

CHANGELOG.md

@@ -52,6 +52,9 @@
 - Fixed the SSBD mitigation not being enabled on `aarch64` with the provided
   `prod-host-setup.md`.
 - Fixed the balloon statistics not working after a snapshot restore event.
+- The `utc_timestamp_ms` now reports the timestamp in ms from the UTC UNIX
+  Epoch, as the name suggests. It was previously using a monotonic clock with
+  an undefined starting point.
 
 ## [0.24.0]
 

src/logger/src/metrics.rs

@@ -769,7 +769,7 @@ struct SerializeToUtcTimestampMs;
 impl Serialize for SerializeToUtcTimestampMs {
     fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
         serializer.serialize_i64(
-            utils::time::get_time_ns(utils::time::ClockType::Monotonic) as i64 / 1_000_000,
+            utils::time::get_time_ns(utils::time::ClockType::Real) as i64 / 1_000_000,
         )
     }
 }

tests/integration_tests/functional/test_metrics.py

@@ -2,8 +2,10 @@
 # SPDX-License-Identifier: Apache-2.0
 """Tests the metrics system."""
 
+import datetime
 import os
 import json
+import math
 import platform
 import host_tools.logging as log_tools
 
@@ -54,4 +56,13 @@ def test_flush_metrics(test_microvm_with_api):
 
     assert set(metrics.keys()) == set(exp_keys)
 
+    utc_time = datetime.datetime.now(datetime.timezone.utc)
+    utc_timestamp_ms = math.floor(utc_time.timestamp() * 1000)
+
+    # Assert that the absolute difference is less than 1 second, to check that
+    # the reported utc_timestamp_ms is actually a UTC timestamp from the Unix
+    # Epoch.Regression test for:
+    # https://github.com/firecracker-microvm/firecracker/issues/2639
+    assert abs(utc_timestamp_ms - metrics['utc_timestamp_ms']) < 1000
+
     microvm.flush_metrics(metrics_fifo)