Add Histogram to empty data points test coverage

[Copilot]

metricsWithoutDataPointsAreNotExported lacked Histogram coverage despite MetricDataFactory.create(HistogramSnapshot) already handling empty data points by returning null.

Changes

• ExportTest.java: Added Histogram to metricsWithoutDataPointsAreNotExported to document and verify the expected no-export behavior alongside the other metric types (Counter, Gauge, Summary, StateSet, Info)

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[Copilot]

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