kamon-system-metrics: Use gauge instead of histogram for memory metrics

[jatcwang]

Does it make more sense to use a gauge rather than histogram for metrics like jvm.memory.used, jvm.memory.free as they aren't high frequency metrics (collected every second by default)?

Will raise a PR if you agree :)

[ivantopo]

We might want to change it in Kamon 3.x because this would be a breaking change. But, assuming that you are asking this because of Prometheus, you could turn those metrics into gauges via config on the Prometheus reporter:

Kamon/reporters/kamon-prometheus/src/main/resources/reference.conf

Lines 80 to 89 in 7f80622

	gauges {
	# Names of the histogram, timer and range sampler metrics that shall be exported as a gauges as well.
	# For each matching metric there will be three gauges representing the current seen distribution (min, max, avg)
	# All patterns are treated as Glob patterns.
	metrics = [
	# example:
	# "http.server*",
	# "*akka*"
	]
	}

[jatcwang]

Cool thanks. For metrics jvm.memory.used I think we want the last polled value instead of min/max/avg, so I guess we'll wait for Kamon 3.