This PR adds counter metrics for the CPU system and the Geth process.
Currently the only metrics available for these items are gauges. Gauges are
fine when the consumer scrapes metrics data at the same interval as Geth
produces new values (every 3 seconds), but it is likely that most consumers
will not scrape that often. Intervals of 10, 15, or maybe even 30 seconds
are probably more common.
So the problem is, how does the consumer estimate what the CPU was doing in
between scrapes. With a counter, it's easy ... you just subtract two
successive values and divide by the time to get a nice, accurate average.
But with a gauge, you can't do that. A gauge reading is an instantaneous
picture of what was happening at that moment, but it gives you no idea
about what was going on between scrapes. Taking an average of values is
meaningless.
The name of a method’s receiver should be a reflection of its identity;
often a one or two letter abbreviation of its type suffices (such as
“c” or “cl” for “Client”). Don’t use generic names such as “me”, “this”
or “self”, identifiers typical of object-oriented languages that place
more emphasis on methods as opposed to functions. The name need not be
as descriptive as that of a method argument, as its role is obvious and
serves no documentary purpose. It can be very short as it will appear
on almost every line of every method of the type; familiarity admits
brevity. Be consistent, too: if you call the receiver “c” in one method,
don’t call it “cl” in another.
