Extend neutron metering agent to generate Granular metering data.
The rationale here is to have data (bytes and packets) not just in
a label basis, but also in tenant, router, and router-label, and tenant-label
basis. This allows operators to develop more complex network monitoring
solutions.
Moreover, I added documentation to explain what is the neutron metering agent,
its configs, and different message formats.
Change-Id: I7b6172f88efd4df89d7bed9a0af52f80c61acbe0
Implements: https://blueprints.launchpad.net/neutron/+spec/granular-metering-data
Closes-Bug: #1886949
bd1467b47c