Fixed Docker CPU Percentage's unusual spike when restarting containers

[Arthur1]

docker.cpuacct_percentage.*.{user,system} metrics observed after restarting a docker container appear to be an unusually high spike.

This problem is due to the fact that the comparison of two uint values is made by subtraction.

mackerel-agent-plugins/mackerel-plugin-docker/lib/docker.go

Lines 273 to 276 in cbd69a9

	userUsage := float64(currentUserUsage.(uint64) - uint64(prevUserUsage.(float64)))
	if userUsage >= 0 {
	(*stats)["docker.cpuacct_percentage."+name+".user"] = userUsage / hostUsage * 100.0 * float64(cpuNumsInt)
	}

This pull request enables to correctly detect that counters have been reset and also set metrics to moderate values in that case.

operational test

before

You can see that docker.cpuacct_percentage.hoge_2e3173.{user,system} metrics have huge values after executing docker restart command.

$ ./build/darwin/arm64/mackerel-plugin-docker
docker.memory.hoge_2e3173.cache	11956224	1692718764
docker.memory.hoge_2e3173.rss	3805184	1692718764
$ ./build/darwin/arm64/mackerel-plugin-docker
docker.cpuacct_percentage.hoge_2e3173.user	2.154874	1692718768
docker.cpuacct_percentage.hoge_2e3173.system	1.628945	1692718768
docker.memory.hoge_2e3173.cache	11956224	1692718768
docker.memory.hoge_2e3173.rss	3805184	1692718768
$ docker restart 2e31732280a7
2e31732280a7
$ ./build/darwin/arm64/mackerel-plugin-docker
docker.cpuacct_percentage.hoge_2e3173.user	246614225174.769592	1692718776
docker.cpuacct_percentage.hoge_2e3173.system	246614225285.428925	1692718776
docker.memory.hoge_2e3173.cache	11923456	1692718776
docker.memory.hoge_2e3173.rss	3735552	1692718776

after

$ ./build/darwin/arm64/mackerel-plugin-docker
docker.memory.hoge_2e3173.cache	11956224	1692717626
docker.memory.hoge_2e3173.rss	3829760	1692717626
$ ./build/darwin/arm64/mackerel-plugin-docker
docker.cpuacct_percentage.hoge_2e3173.user	3.126744	1692717634
docker.cpuacct_percentage.hoge_2e3173.system	2.455736	1692717634
docker.memory.hoge_2e3173.cache	11956224	1692717634
docker.memory.hoge_2e3173.rss	3829760	1692717634
$ docker restart 2e31732280a7
2e31732280a7
$ ./build/darwin/arm64/mackerel-plugin-docker
docker.memory.hoge_2e3173.cache	11923456	1692717644
docker.memory.hoge_2e3173.rss	3710976	1692717644
docker.cpuacct_percentage.hoge_2e3173.user	1.112812	1692717644
docker.cpuacct_percentage.hoge_2e3173.system	1.229388	1692717644

[ne-sachirou]

🙆‍♀️