NIFI-15327 Enabled TestSiteToSiteMetricsReportingTask on Windows.

[dan-s1]

Summary

NIFI-15327
The reason there was a test failure on Windows was due to line 144 in MetricsService
where method fileDescriptorUsage() of JvmMetrics returned NaN.

Apparently it returned NaN because "File descriptor usage" is a metric specific to Unix-like operating systems. When trying to translate NaN to JSON, org.eclipse.parsson.JsonNumberImpl.getJsonNumber threw an exception since according to the official JSON specification, NaN is not valid numeric values within a strict JSON document.
Hence the solution for this was to test for NaN and if present replace the value with -1 (a valid numeric value and a number which indicates more than 0 that there is no possibility of any File descriptors).
In addition to removing the disabled on Windows annotation in TestSiteToSiteMetricsReportingTask, I also followed the Intellij suggestions in order to clean up the code.

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