Return {runName: {tagName: {displayName: ..., description: ...}}}.

def index_impl(self):
    """Return {runName: {tagName: {displayName: ..., description: ...}}}."""
    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