Plugin.EvaluateThreshold reduces severity of plugin exit status code on later perfdata evaluations

[atc0005]

While working on #278 I reviewed the logic for this method:

go-nagios/range.go

Line 147 in d9dbfd9

func (p *Plugin) EvaluateThreshold(perfData ...PerformanceData) error {

and it appears that the current logic may have a bug.

From what I can tell the method:

• does not sort the collection before evaluation
• does not use a high-water mark check to determine if the highest exit code state has been reached

This can lead to a performance data metric warranting a CRITICAL exit code being ignored if a later performance data metric in the input collection is evaluated which indicates that a WARNING exit code should be set.

Whether a single plugin should support checking multiple separate thresholds, each of which can lead to having separate state evaluations is something to consider but is likely supported (unverified) by stock / standard plugins.