NPD needs significant unit test coverage

[DigitalVeer]

We should improve the overall test coverage present in NPD.

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Currently, NPD's unit test coverage is 46.0% of 2299 statements. There are important packages, such as the custom plugin monitor, that have close to no test coverage at all.

Recent PRs such as #1322 and #1323 showed real headroom to make NPD faster and memory effecient, and more optimization work like that is worth pursuing. NPD also stands to benefit from agent-assisted development which can help us unlock more performance and reliability improvements over time. This type of work is only safe on top of reliable test infrastructure.

The table below are packages worth targeting in a first round of unit test improvement due to their importance in NPD workloads:

Package	Coverage	Planned work
pkg/exporters/k8sexporter	0.0%	export test with the fake problem client
pkg/exporters/prometheusexporter	0.0%	export test
pkg/custompluginmonitor	0.8%	table test for generateStatus
pkg/healthchecker	20.6%	tests for getRepairFunc, getUptimeFunc, checkForPattern
pkg/systemlogmonitor/logwatchers/journald	23.2%	tests for the watch loop
pkg/systemstatsmonitor	45.2%	fixture test for the OS feature collector

Every other measured package is in the collapsed table. A lot of the remaining 0% rows are thin wrappers, build-time metadata, or type declarations where a unit test would not help us much immediately.

Full package test coverage

Package	Coverage
pkg/exporters/stackdriver/gce	0.0%
pkg/systemlogmonitor/logwatchers	0.0%
pkg/systemlogmonitor/logwatchers/testing	0.0%
pkg/version	0.0%
cmd/healthchecker	0.0%
cmd/logcounter	0.0%
cmd/logcounter/options	0.0%
pkg/exporters/stackdriver	3.3%
pkg/exporters/k8sexporter/problemclient	5.4%
pkg/problemdetector	23.3%
cmd/healthchecker/options	26.5%
cmd/nodeproblemdetector	36.4%
pkg/util/metrics	44.2%
pkg/problemdaemon	45.0%
pkg/systemlogmonitor	52.2%
cmd/options	55.9%
pkg/exporters	56.2%
pkg/logcounter	57.9%
pkg/util	62.5%
pkg/systemstatsmonitor/types	67.6%
pkg/util/metrics/system	77.9%
pkg/systemlogmonitor/logwatchers/filelog	78.6%
pkg/exporters/k8sexporter/condition	78.9%
pkg/systemlogmonitor/logwatchers/kmsg	79.7%
pkg/custompluginmonitor/plugin	83.5%
pkg/problemmetrics	85.3%
pkg/custompluginmonitor/types	94.8%
pkg/healthchecker/types	95.6%
pkg/exporters/stackdriver/config	100.0%
pkg/util/tomb	100.0%
pkg/types	no tests
pkg/systemlogmonitor/types	no tests
pkg/systemlogmonitor/logwatchers/types	no tests
pkg/logcounter/types	no tests
cmd/nodeproblemdetector/exporterplugins	no tests
cmd/nodeproblemdetector/problemdaemonplugins	no tests

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The largest gaps exist in the custom plugin monitor, the current exporters, and the health checker, and we should target those first.

/sig node
/kind cleanup

[DigitalVeer]

/assign