Add constructor to initialize plugin start time, provide `time` Performance Data metric "automatically"

[atc0005]

Example code snippet from the atc0005/check-vmware project:

func main() {

    // Start the timer. We'll use this to emit the plugin runtime as a
    // performance data metric.
    pluginStart := time.Now()

    // Set initial "state" as valid, adjust as we go.
    var nagiosExitState = nagios.ExitState{
        LastError:      nil,
        ExitStatusCode: nagios.StateOKExitCode,
    }

    // defer this from the start so it is the last deferred function to run
    defer nagiosExitState.ReturnCheckResults()

    // Collect last minute details just before ending plugin execution.
    defer func(exitState *nagios.ExitState, start time.Time) {

        // Record plugin runtime, emit this metric regardless of exit
        // point/cause.
        runtimeMetric := nagios.PerformanceData{
            Label: "time",
            Value: fmt.Sprintf("%dms", time.Since(start).Milliseconds()),
        }
        if err := exitState.AddPerfData(false, runtimeMetric); err != nil {
            zlog.Error().
                Err(err).
                Msg("failed to add time (runtime) performance data metric")
        }

        // Annotate errors (if applicable) with additional context to aid in
        // troubleshooting.
        nagiosExitState.LastError = vsphere.AnnotateError(nagiosExitState.LastError)
    }(&nagiosExitState, pluginStart)

Some thoughts come to mind:

• add an unexported field (runtime ?) to track plugin runtime
• create a constructor to initialize this field
• add time Performance Data metric automatically
  • likely via the ReturnCheckResults() method
  • this would provide this metric "for free" for each dependent project

The initialization logic for plugins in the atc0005/check-vmware project would be greatly simplified, and presumably other projects would benefit from these changes.

One "gotcha" that I'd need to guard against would be if client code opts to not use the constructor: the ReturnCheckResults() method would need to assert that the runtime field is not its zero value before generating a performance data metric.

[atc0005]

Implemented via GH-167.