RFC: NetworkManager Device Statistics

[Tony Espy <espy canonical com>]

We have a new requirement to add a data activity indicator to Ubuntu 
Touch for certification in certain markets.

We've had some internal debate about whether or not we should extend 
NetworkManager to provide this information to the rest of the system vs. 
adding logic directly to our touch-specific network indicator.

The current prototype is based on a similar facility in Connman, and 
involves RTM_GETLINK polling to retrieve device-specific Rx and Tx 
statistics.

Connman's API can be seen here:

https://git.kernel.org/cgit/network/connman/connman.git/tree/doc/counter-api.txt
https://git.kernel.org/cgit/network/connman/connman.git/tree/doc/manager-api.txt#n90

Our proposed implementation involves extending 
NMPlatform/NMLinuxPlatform and adding some new properties to the 
NetworkManager.Device interface:

* TxBytes, TxPackets, [calculated TxRate (byte/s)] and Rx equivalents
* EnableStatProps : boolean property to enable/disable device statistics
* StatPropsRefreshRateMs : refresh rate for Tx/Rx properties in milliseconds

An alternative to extending the Device interface would be adding a 
dedicated Device.Statistics interface; and perhaps a Device interface 
method ( vs. the EnableStatProps property ) to control 
enabling/disabling collection of statistics on a per-device basis.

The idea behind EnableStatProps is to avoid overloading the system with 
PropertiesChanged signals. The Tx/Rx properties will not refresh if 
EnableStatProps is false, and will refresh, sending PropertiesChanged 
signals with the rate specified in StatPropsRefreshRateMs, when true.
In our case, we'd explicitly disable statistics when the screen on a 
device is turned off.

Thoughts?

Regards,
/tony