add neutron-metering-agnet on neutron chart

This blueprint proposes adding a neutron-metering-agent deployment on neutron chart.

A neutron temptest check that metering is included in extension list in default.
Though we can make skip the test list using blacklist but I think it's better to add metering-agent to our neutron chart.
and by making it optional, so We can select to deploy it or not.

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tempest.api.network.test_extensions.ExtensionsTestJSON.test_list_show_extensions
tempest.api.network.test_extensions.py [line 36~41]
        expected_alias = ['security-group', 'l3_agent_scheduler',
                          'ext-gw-mode', 'binding', 'quotas',
                          'agent', 'dhcp_agent_scheduler', 'provider',
                          'router', 'extraroute', 'external-net',
                          'allowed-address-pairs', 'extra_dhcp_opt',
                          'metering', 'dvr']
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