Formatted output always reports 0 for initial tx/rx rate

[SLongofono]

Issue Type:
Bug report

What happened:
Using formatted output along with quitafter always incorrectly returns 0 traffic for the first output reported, regardless of the sampling interval or the number of outputs specified via quitafter.

Steps to reproduce:
Monitor an interface with active connections, for example 'eth*'.
Issue the command:

bmon -p eth* -o format:'fmt=$(element:name)\t$(attr:rxrate:bytes)\t$(attr:txrate:bytes)\n;quitafter=5'

If the rate interval is adjust downward, this behavior tends to appear on more of the outputs. For example, at a rate of 0.05, the first 3 outputs show 0 traffic on all interfaces.

What was the expected behavior:
The library should report only traffic it has actually observed. The fact that this behavior occurs on more than the first output as the rate interval is reduced suggests that this bug is related to not having enough time to make a traffic observation. Reporting 0 traffic because there has not been enough time to make an observation is misleading, and arguably worse than not reporting at all.