[shakuzen]

HdrHistogram is not exposed in the API. It is an implementation detail of client-side percentiles. The scope of the dependency can be changed accordingly.

This was found while working on #1599.

[shakuzen]

I may have been too eager with this change. HdrHistogram's DoubleHistogram and DoubleRecorder is part of the (package-private) API in the public class TimeWindowPercentileHistogram:

micrometer/micrometer-core/src/main/java/io/micrometer/core/instrument/distribution/TimeWindowPercentileHistogram.java

Lines 42 to 65 in 4f3cf5a

	@Override
	DoubleRecorder newBucket() {
	return new DoubleRecorder(percentilePrecision(distributionStatisticConfig));
	}
	@Override
	void recordDouble(DoubleRecorder bucket, double value) {
	bucket.recordValue(value);
	}
	@Override
	void recordLong(DoubleRecorder bucket, long value) {
	bucket.recordValue(value);
	}
	@Override
	void resetBucket(DoubleRecorder bucket) {
	bucket.reset();
	}
	@Override
	DoubleHistogram newAccumulatedHistogram(DoubleRecorder[] ringBuffer) {
	return new DoubleHistogram(percentilePrecision(distributionStatisticConfig));
	}

Though I'm not sure if that is a problem for changing the scope here or not. Outside of the local package, I don't think there is reference to any HdrHistogram classes in API that users would be able to compile against.

[shakuzen]

With #3325 in now, I think we can give this a try in 1.10. User code should not compile against Micrometer API that has the HdrHistogram API in it with normal usage that I can think of. If client-side percentiles are being used, HdrHistogram is still needed at runtime. This pull request still declares HdrHistogram as needed at runtime.