code: def index_impl(self):
      
      if self._db_connection_provider:
        # Read tags from the database.
        db = self._db_connection_provider()
        cursor = db.execute('''
          SELECT
            Tags.tag_name,
            Tags.display_name,
            Runs.run_name
          FROM Tags
          JOIN Runs
            ON Tags.run_id = Runs.run_id
          WHERE
            Tags.plugin_name = ?
        ''', (metadata.PLUGIN_NAME,))
        result = collections.defaultdict(dict)
        for row in cursor:
          tag_name, display_name, run_name = row
          result[run_name][tag_name] = {
              'displayName': display_name,
              # TODO(chihuahua): Populate the description. Currently, the tags
              # table does not link with the description table.
              'description': '',
          }
        return result
  
      runs = self._multiplexer.Runs()
      result = {run: {} for run in runs}
  
      mapping = self._multiplexer.PluginRunToTagToContent(metadata.PLUGIN_NAME)
      for (run, tag_to_content) in six.iteritems(mapping):
        for (tag, content) in six.iteritems(tag_to_content):
          content = metadata.parse_plugin_metadata(content)
          summary_metadata = self._multiplexer.SummaryMetadata(run, tag)
          result[run][tag] = {'displayName': summary_metadata.display_name,
                              'description': plugin_util.markdown_to_safe_html(
                                  summary_metadata.summary_description)}
  
      return result
docs: Return {runName: {tagName: {displayName: ..., description: ...}}}.