class_docstr: A TF step metric that emits multiple values per step.
  
  The only difference between `TFSTepMetric` and `TFMultiMetricStepMetric` is
  that the latter creates at each step many scalar summaries, one per metric.
code: def metric_names(self):
      return self._metric_names
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import tensorflow
  from tf_agents.utils import common
label_desc: Use image node_id 2 for calling the TFMultiMetricStepMetric obj's underlying member method code with example usage: obj.metric_names() and returns: self