==================
- Rework ``PrometheusExporterScript`` class, using ``click`` for command line
parsing.
- Rework logging based on ``structlog``, support structured logging output as
JSON.
- Support reading configuration options from environment variables.
- Support reading configuration from `.env` file.
**Note**: this version contains incompatible changes with the ``2.x`` series:
- Command-line parsing is now provided by the ``click`` library. To provide
additional command-line options, the ``command_line_parameters()`` method
should be implemented, returning a list of ``click.Parameter`` subclasses
(e.g. ``click.Option`` or ``click.Argument``)
- The ``structlog`` library is used for logging, providing support for
structured logging. The logger API is essentially backwards compatible with
the builtin ``logging`` module, with the added ability of passing keyword
arguments for structured logging.