refactor: move all datetime.now() calls to UTC (#1589)

# What does this PR do?

Updated all instances of datetime.now() to use timezone.utc for
consistency in handling time across different systems. This ensures that
timestamps are always in Coordinated Universal Time (UTC), avoiding
issues with time zone discrepancies and promoting uniformity in
time-related data.

Signed-off-by: Sébastien Han <seb@redhat.com>

llama_stack/providers/utils/telemetry/tracing.py

@@ -11,7 +11,7 @@ import logging
 import queue
 import threading
 import uuid
-from datetime import datetime
+from datetime import datetime, timezone
 from functools import wraps
 from typing import Any, Callable, Dict, List, Optional
 
@@ -86,7 +86,7 @@ class TraceContext:
             span_id=generate_short_uuid(),
             trace_id=self.trace_id,
             name=name,
-            start_time=datetime.now(),
+            start_time=datetime.now(timezone.utc),
             parent_span_id=current_span.span_id if current_span else None,
             attributes=attributes,
         )
@@ -203,7 +203,7 @@ class TelemetryHandler(logging.Handler):
             UnstructuredLogEvent(
                 trace_id=span.trace_id,
                 span_id=span.span_id,
-                timestamp=datetime.now(),
+                timestamp=datetime.now(timezone.utc),
                 message=self.format(record),
                 severity=severity(record.levelname),
             )