Easily save linear histograms

[brendangregg]

A follow on from #144, this would allow a linear histogram to be easily saved. Eg:

BPF_HIST(myhist, linear, 0, 100, 10);
[...]
    myhist.increment(value);

Where the start is 0, end is 100, and a step of 10.

More exotic histograms can already be coded using BPF_TABLE directly. It's not too hard to do linear either, this is just for minor convenience.

[stefreak]

This would bring great value for everyone who tries to write a prometheus exporter based on BCC, because Prometheus expects histogram data to have a linear distribution for calculation of quantiles (see also here: https://prometheus.io/docs/prometheus/latest/querying/functions/#histogram_quantile() The histogram_quantile() function interpolates quantile values by assuming a linear distribution within a bucket.)

[hharnisc]

Hey @brendangregg was looking for a good first issue to start contributing and came across this one. Is this at all related to https://github.com/iovisor/bcc/blob/master/src/python/bcc/table.py#L346

[brendangregg]

Looks like I did it in 39cece4 (so that I could add runqlen.py) but I forgot to close this ticket. Closing now!