How to use the gnocchi.service.prepare_service function in gnocchi

def metricd():
    conf = cfg.ConfigOpts()
    conf.register_cli_opts([
        cfg.IntOpt("stop-after-processing-metrics",
                   default=0,
                   min=0,
                   help="Number of metrics to process without workers, "
                   "for testing purpose"),
    ])
    conf = service.prepare_service(conf=conf)

    if conf.stop_after_processing_metrics:
        metricd_tester(conf)
    else:
        MetricdServiceManager(conf).run()

def prepare_service(conf=None):
    if conf is None:
        conf = cfg.ConfigOpts()

    opts.set_defaults()
    policy_opts.set_defaults(conf)
    conf = service.prepare_service(conf=conf)
    cfg_path = conf.oslo_policy.policy_file
    if not os.path.isabs(cfg_path):
        cfg_path = conf.find_file(cfg_path)
    if cfg_path is None or not os.path.exists(cfg_path):
        cfg_path = os.path.abspath(os.path.join(os.path.dirname(__file__),
                                   '..', 'rest', 'policy.json'))
    conf.set_default('policy_file', cfg_path, group='oslo_policy')
    return conf

def injector():
    conf = cfg.ConfigOpts()
    conf.register_cli_opts([
        cfg.IntOpt("metrics", default=1, min=1),
        cfg.StrOpt("archive-policy-name", default="low"),
        cfg.StrOpt("creator", default="admin"),
        cfg.IntOpt("batch-of-measures", default=1000),
        cfg.IntOpt("measures-per-batch", default=10),
    ])
    conf = service.prepare_service(conf=conf)
    index = indexer.get_driver(conf)
    instore = incoming.get_driver(conf)

    def todo():
        metric = index.create_metric(
            uuid.uuid4(),
            creator=conf.creator,
            archive_policy_name=conf.archive_policy_name)

        for _ in six.moves.range(conf.batch_of_measures):
            measures = [
                incoming.Measure(
                    utils.dt_in_unix_ns(utils.utcnow()), random.random())
                for __ in six.moves.range(conf.measures_per_batch)]
            instore.add_measures(metric, measures)

def upgrade():
    conf = cfg.ConfigOpts()
    sack_number_opt = copy.copy(_SACK_NUMBER_OPT)
    sack_number_opt.default = 128
    conf.register_cli_opts([
        cfg.BoolOpt("skip-index", default=False,
                    help="Skip index upgrade."),
        cfg.BoolOpt("skip-storage", default=False,
                    help="Skip storage upgrade."),
        cfg.BoolOpt("skip-incoming", default=False,
                    help="Skip incoming storage upgrade."),
        cfg.BoolOpt("skip-archive-policies-creation", default=False,
                    help="Skip default archive policies creation."),
        sack_number_opt,
    ])
    conf = service.prepare_service(conf=conf, log_to_std=True)
    if not conf.skip_index:
        index = indexer.get_driver(conf)
        LOG.info("Upgrading indexer %s", index)
        index.upgrade()
    if not conf.skip_storage:
        s = storage.get_driver(conf)
        LOG.info("Upgrading storage %s", s)
        s.upgrade()
    if not conf.skip_incoming:
        i = incoming.get_driver(conf)
        LOG.info("Upgrading incoming storage %s", i)
        i.upgrade(conf.sacks_number)

    if (not conf.skip_archive_policies_creation
            and not index.list_archive_policies()
            and not index.list_archive_policy_rules()):

with connectable.connect() as connection:
            context.configure(
                connection=connection,
                target_metadata=target_metadata
            )

            with context.begin_transaction():
                context.run_migrations()

    indexer.disconnect()

# If `alembic' was used directly from the CLI
if not hasattr(config, "conf"):
    from gnocchi import service
    config.conf = service.prepare_service([])

if context.is_offline_mode():
    run_migrations_offline()
else:
    run_migrations_online()

def change_sack_size():
    conf = cfg.ConfigOpts()
    conf.register_cli_opts([_SACK_NUMBER_OPT])
    conf = service.prepare_service(conf=conf, log_to_std=True)
    s = incoming.get_driver(conf)
    try:
        report = s.measures_report(details=False)
    except incoming.SackDetectionError:
        LOG.error('Unable to detect the number of storage sacks.\n'
                  'Ensure gnocchi-upgrade has been executed.')
        return
    remainder = report['summary']['measures']
    if remainder:
        LOG.error('Cannot change sack when non-empty backlog. Process '
                  'remaining %s measures and try again', remainder)
        return
    LOG.info("Removing current %d sacks", s.NUM_SACKS)
    s.remove_sacks()
    LOG.info("Creating new %d sacks", conf.sacks_number)
    s.upgrade(conf.sacks_number)

def injector():
    conf = cfg.ConfigOpts()
    conf.register_cli_opts([
        cfg.IntOpt("--measures",
                   help="Measures per metric."),
        cfg.IntOpt("--metrics",
                   help="Number of metrics to create."),
        cfg.IntOpt("--archive-policy-name",
                   help="Name of archive policy to use.",
                   default="low"),
        cfg.IntOpt("--interval",
                   help="Interval to sleep between metrics sending."),
        cfg.BoolOpt("--process", default=False,
                    help="Process the ingested measures."),
    ])
    return _inject(service.prepare_service(conf=conf, log_to_std=True),
                   metrics=conf.metrics,
                   measures=conf.measures,
                   archive_policy_name=conf.archive_policy_name,
                   process=conf.process,
                   interval=conf.interval)