docstr: Returns a single image's blob_key after it has been written.
code: def _get_image_blob_key(self, run, tag, step=0, sample=0):
      """Returns a single image's blob_key after it has been written."""
      mapping = self._plugin._data_provider.read_blob_sequences(
          context.RequestContext(),
          experiment_id="expid",
          plugin_name=image_metadata.PLUGIN_NAME,
          downsample=10,
          run_tag_filter=provider.RunTagFilter(tags=[tag]),
      )
      blob_sequence_datum = mapping[run][tag][step]
      # For images, the first 2 datum values are ignored.
      return blob_sequence_datum.values[2 + sample].blob_key
  
imports: import argparse
  import collections.abc
  import os.path
  import tensorflow.compat.v1
  import tensorflow.compat.v2
  from tensorboard import context
  from tensorboard.backend.event_processing import data_provider
  from tensorboard.backend.event_processing import plugin_event_multiplexer
  from tensorboard.data import provider
  from tensorboard.plugins import base_plugin
  from tensorboard.plugins.image import metadata
  from tensorboard.plugins.metrics import metrics_plugin
label_desc: Use image node_id 8 for calling the MetricsPluginTest obj's underlying member method code with example usage: obj._get_image_blob_key(run, tag, step, sample) and returns: blob_sequence_datum