Protect start_time and end_time from bein set manually by the user

Fixes open-telemetry#361

Author: ocelotl

opentelemetry-sdk/src/opentelemetry/sdk/trace/__init__.py

@@ -171,10 +171,18 @@ def __init__(
         else:
             self.links = BoundedList.from_seq(MAX_NUM_LINKS, links)
 
-        self.end_time = None  # type: Optional[int]
-        self.start_time = None  # type: Optional[int]
+        self._end_time = None  # type: Optional[int]
+        self._start_time = None  # type: Optional[int]
         self.instrumentation_info = instrumentation_info
 
+    @property
+    def start_time(self):
+        return self._start_time
+
+    @property
+    def end_time(self):
+        return self._end_time
+
     def __repr__(self):
         return '{}(name="{}", context={})'.format(
             type(self).__name__, self.name, self.context
@@ -243,7 +251,7 @@ def start(self, start_time: Optional[int] = None) -> None:
                 return
             has_started = self.start_time is not None
             if not has_started:
-                self.start_time = (
+                self._start_time = (
                     start_time if start_time is not None else time_ns()
                 )
         if has_started:
@@ -259,7 +267,7 @@ def end(self, end_time: Optional[int] = None) -> None:
                 raise RuntimeError("Calling end() on a not started span.")
             has_ended = self.end_time is not None
             if not has_ended:
-                self.end_time = end_time if end_time is not None else time_ns()
+                self._end_time = end_time if end_time is not None else time_ns()
         if has_ended:
             logger.warning("Calling end() on an ended span.")
             return