Automatically update RRD max values for port(channel)s

For Portchannels with high throughput the default max value of 100Gigabit isn't sufficient anymore.

Consider updating the max value for each port to the advertised interface speed during each poller run. The operation should be sufficiently cheap to not cause any problems.

IRC conversation for reference:

13:48 < Robe> observium apparently created a port channel with a max of 100gbit albeit the channel having 160gbit
13:48 < Robe> is it safe to just fix the rrd file by hand or will this be overwritten by subsequent updates?
13:52 < sid3windr> Robe: we only specify that upon rrd creation
13:52 < sid3windr> so yes, you can fix it
13:52 < sid3windr> and we may need a solution for this :[
13:53 < sid3windr> job also runs into this regularly with his port channels I believe
13:53 < sid3windr> :[
13:54 < Robe> well
13:54 < Robe> bumping the hardcoded limit to 200gigabit and you should be good to go!
13:54 < Robe> (till next year)
13:54 < sid3windr> exactly
13:54 < sid3windr>
13:54 < Robe> otherwise you could expose a DS tuning knob to the specific interface views?
13:55 < sid3windr> but it is/was supposed to be so you don't get ridiculous peaks on your FE interface :/
13:55 < sid3windr> well
13:55 < Robe> and/or alert if a advertised port speed exceeds the configured max
13:55 < sid3windr> yea
14:00 < sid3windr> but is it easy to change a max ds value in an rrd?
14:01 < Robe> rrdtool tune port-1000001.rrd --maximum OUTOCTETS:2.5000000000e+10
14:01 < Robe> (same for inoctets)
14:01 < sid3windr> aha
14:01 < sid3windr> does that take long?
14:02 < Robe> four milliseconds on my box
14:02 < sid3windr> we could maybe, upon discovery of a new port, use its speed, and if there's a change in the field, update the rrd
14:02 < Robe> apparently rrdtool mmaps the file, changes the field and closes the file again
14:02 < Robe> seems to be even cheaper than inserting new values
14:03 < Robe> sid3windr: that would be great
14:03 < Robe> especially with the arista port channels the speed dynamically updates reflecting the number of active LAG members