Add "LOGPOINT: " prefix to Python logpoints.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123672037

Author: b-daniels

src/googleclouddebugger/capture_collector.py

@@ -467,7 +467,7 @@ def Log(self, frame):
         self._definition.get('logMessageFormat', ''),
         self._EvaluateExpressions(frame))
 
-    self._log_message(message)
+    self._log_message('LOGPOINT: ' + message)
     return None
 
   def _EvaluateExpressions(self, frame):