Improves the metering doc. Agent/Plugin conf.

Add the metering agent to the list of network agents table.
Also add a section to specify the service plugin which needs
to be used to enable the metering on plugin side.

backport: havana
Partial-bug: #1202967
Change-Id: If12b655e3459cc0ae968fce4189db6623d2c3921
Merge "Improves the metering doc. Agent/Plugin conf."