code: def mongo_obs_without_prefix():
      # create a mongo observer without a collection prefix
      # i.e. should default to collections 'runs' and 'metrics'
      real_client = pymongo.MongoClient()
      return MongoObserver(client=real_client)
  
imports: import datetime
  import os
  import sys
  from glob import glob
  import mock
  import pytest
  from sacred.metrics_logger import ScalarMetricLogEntry, linearize_metrics
  import gridfs
  from mongomock.gridfs import enable_gridfs_integration
  from .failing_mongo_mock import FailingMongoClient
  from sacred.dependencies import get_digest
  from sacred.observers.mongo import MongoObserver, force_bson_encodeable
label_desc: Use image node_id 6 for calling a global function with example usage: mongo_obs_without_prefix() and returns: MongoObserver