fix: Use monotonic clock to compute durations

sentry_sdk/tracing.py

@@ -1,8 +1,9 @@
 import re
 import uuid
 import contextlib
+import time
 
-from datetime import datetime
+from datetime import datetime, timedelta
 
 import sentry_sdk
 
@@ -101,6 +102,7 @@ class Span(object):
         "op",
         "description",
         "start_timestamp",
+        "_start_timestamp_monotonic",
         "timestamp",
         "_tags",
         "_data",
@@ -134,6 +136,14 @@ def __init__(
         self._tags = {}  # type: Dict[str, str]
         self._data = {}  # type: Dict[str, Any]
         self.start_timestamp = datetime.utcnow()
+        try:
+            # TODO: For Python 3.7+, we could use a clock with ns resolution:
+            # self._start_timestamp_monotonic = time.perf_counter_ns()
+
+            # Python 3.3+
+            self._start_timestamp_monotonic = time.perf_counter()
+        except AttributeError:
+            pass
 
         #: End timestamp of span
         self.timestamp = None  # type: Optional[datetime]
@@ -309,7 +319,11 @@ def finish(self, hub=None):
             # This transaction is already finished, so we should not flush it again.
             return None
 
-        self.timestamp = datetime.utcnow()
+        try:
+            duration_seconds = time.perf_counter() - self._start_timestamp_monotonic
+            self.timestamp = self.start_timestamp + timedelta(seconds=duration_seconds)
+        except AttributeError:
+            self.timestamp = datetime.utcnow()
 
         _maybe_create_breadcrumbs_from_span(hub, self)