This plugin is active iff any run has at least one histograms tag.

def is_active(self):
    """This plugin is active iff any run has at least one histograms tag."""
    if self._db_connection_provider:
      # The plugin is active if one relevant tag can be found in the database.
      db = self._db_connection_provider()
      cursor = db.execute('''
        SELECT
          1
        FROM Tags
        WHERE Tags.plugin_name = ?
        LIMIT 1
      ''', (metadata.PLUGIN_NAME,))
      return bool(list(cursor))

    return bool(self._multiplexer) and any(self.index_impl().values())