Export available entropy

[RichiH]

It would be nice if /proc/sys/kernel/random/entropy_avail was exported as well.

Opinions?

[brian-brazil]

Seems reasonable.

[grobie]

https://media.ccc.de/v/32c3-7441-the_plain_simple_reality_of_entropy#video ;)

[RichiH]

RichiH@c696546 does not add the correct values, but is as far as I got today.

And yes, it's the first time I tried doing anything in Go.

[RichiH]

Please see https://github.com/RichiH/node_exporter/tree/playground/entropy

Open issues:

• Cleanup
• Even if I put 1338 into collector/fixtures/proc/sys/kernel/random/entropy_avail the test passes
  • Negative tests might be a good idea to catch such things
• readUintFromFile() should probably be factored out into someplace else
• @pborzenkov might want to consider switching Add new collector exposing 'ksmd' stats #165 onto the same function; it's where I stole my initial version and the current test code from

[RichiH]

#190