Add configuration support for ExponentialHistogram in OTLP Registry

Closes gh-41837

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/export/otlp/OtlpProperties.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,6 +20,7 @@ import java.util.Map;
 import java.util.concurrent.TimeUnit;
 
 import io.micrometer.registry.otlp.AggregationTemporality;
+import io.micrometer.registry.otlp.HistogramFlavor;
 
 import org.springframework.boot.actuate.autoconfigure.metrics.export.properties.StepRegistryProperties;
 import org.springframework.boot.context.properties.ConfigurationProperties;
@@ -57,6 +58,22 @@ public class OtlpProperties extends StepRegistryProperties {
 	 */
 	private Map<String, String> headers;
 
+	/**
+	 * Histogram type to be preferred when histogram publishing is enabled.
+	 */
+	private HistogramFlavor histogramFlavor = HistogramFlavor.EXPLICIT_BUCKET_HISTOGRAM;
+
+	/**
+	 * Max scale to use for exponential histograms, if configured.
+	 */
+	private int maxScale = 20;
+
+	/**
+	 * Maximum number of buckets to be used for exponential histograms, if configured.
+	 * This has no effect on explicit bucket histograms.
+	 */
+	private int maxBucketCount = 160;
+
 	/**
 	 * Time unit for exported metrics.
 	 */
@@ -97,6 +114,30 @@ public class OtlpProperties extends StepRegistryProperties {
 		this.headers = headers;
 	}
 
+	public HistogramFlavor getHistogramFlavor() {
+		return this.histogramFlavor;
+	}
+
+	public void setHistogramFlavor(HistogramFlavor histogramFlavor) {
+		this.histogramFlavor = histogramFlavor;
+	}
+
+	public int getMaxScale() {
+		return this.maxScale;
+	}
+
+	public void setMaxScale(int maxScale) {
+		this.maxScale = maxScale;
+	}
+
+	public int getMaxBucketCount() {
+		return this.maxBucketCount;
+	}
+
+	public void setMaxBucketCount(int maxBucketCount) {
+		this.maxBucketCount = maxBucketCount;
+	}
+
 	public TimeUnit getBaseTimeUnit() {
 		return this.baseTimeUnit;
 	}

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/metrics/export/otlp/OtlpPropertiesConfigAdapter.java

@@ -22,6 +22,7 @@ import java.util.Map;
 import java.util.concurrent.TimeUnit;
 
 import io.micrometer.registry.otlp.AggregationTemporality;
+import io.micrometer.registry.otlp.HistogramFlavor;
 import io.micrometer.registry.otlp.OtlpConfig;
 
 import org.springframework.boot.actuate.autoconfigure.metrics.export.properties.StepRegistryPropertiesConfigAdapter;
@@ -98,6 +99,21 @@ class OtlpPropertiesConfigAdapter extends StepRegistryPropertiesConfigAdapter<Ot
 		return get(OtlpProperties::getHeaders, OtlpConfig.super::headers);
 	}
 
+	@Override
+	public HistogramFlavor histogramFlavor() {
+		return get(OtlpProperties::getHistogramFlavor, OtlpConfig.super::histogramFlavor);
+	}
+
+	@Override
+	public int maxScale() {
+		return get(OtlpProperties::getMaxScale, OtlpConfig.super::maxScale);
+	}
+
+	@Override
+	public int maxBucketCount() {
+		return get(OtlpProperties::getMaxBucketCount, OtlpConfig.super::maxBucketCount);
+	}
+
 	@Override
 	public TimeUnit baseTimeUnit() {
 		return get(OtlpProperties::getBaseTimeUnit, OtlpConfig.super::baseTimeUnit);

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/metrics/export/otlp/OtlpPropertiesConfigAdapterTests.java

@@ -21,6 +21,7 @@ import java.util.Map;
 import java.util.concurrent.TimeUnit;
 
 import io.micrometer.registry.otlp.AggregationTemporality;
+import io.micrometer.registry.otlp.HistogramFlavor;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
@@ -85,6 +86,39 @@ class OtlpPropertiesConfigAdapterTests {
 		assertThat(createAdapter().headers()).containsEntry("header", "value");
 	}
 
+	@Test
+	void whenPropertiesHistogramFlavorIsNotSetAdapterHistogramFlavorReturnsExplicitBucketHistogram() {
+		assertThat(createAdapter().histogramFlavor()).isSameAs(HistogramFlavor.EXPLICIT_BUCKET_HISTOGRAM);
+	}
+
+	@Test
+	void whenPropertiesHistogramFlavorIsSetAdapterHistogramFlavorReturnsIt() {
+		this.properties.setHistogramFlavor(HistogramFlavor.BASE2_EXPONENTIAL_BUCKET_HISTOGRAM);
+		assertThat(createAdapter().histogramFlavor()).isSameAs(HistogramFlavor.BASE2_EXPONENTIAL_BUCKET_HISTOGRAM);
+	}
+
+	@Test
+	void whenPropertiesMaxScaleIsNotSetAdapterMaxScaleReturns20() {
+		assertThat(createAdapter().maxScale()).isEqualTo(20);
+	}
+
+	@Test
+	void whenPropertiesMaxScaleIsSetAdapterMaxScaleReturnsIt() {
+		this.properties.setMaxScale(5);
+		assertThat(createAdapter().maxScale()).isEqualTo(5);
+	}
+
+	@Test
+	void whenPropertiesMaxBucketCountIsNotSetAdapterMaxBucketCountReturns160() {
+		assertThat(createAdapter().maxBucketCount()).isEqualTo(160);
+	}
+
+	@Test
+	void whenPropertiesMaxBucketCountIsSetAdapterMaxBucketCountReturnsIt() {
+		this.properties.setMaxBucketCount(6);
+		assertThat(createAdapter().maxBucketCount()).isEqualTo(6);
+	}
+
 	@Test
 	void whenPropertiesBaseTimeUnitIsNotSetAdapterBaseTimeUnitReturnsMillis() {
 		assertThat(createAdapter().baseTimeUnit()).isSameAs(TimeUnit.MILLISECONDS);

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/metrics/export/otlp/OtlpPropertiesTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -37,6 +37,9 @@ class OtlpPropertiesTests extends StepRegistryPropertiesTests {
 		assertStepRegistryDefaultValues(properties, config);
 		assertThat(properties.getUrl()).isEqualTo(config.url());
 		assertThat(properties.getAggregationTemporality()).isSameAs(config.aggregationTemporality());
+		assertThat(properties.getHistogramFlavor()).isSameAs(config.histogramFlavor());
+		assertThat(properties.getMaxScale()).isEqualTo(config.maxScale());
+		assertThat(properties.getMaxBucketCount()).isEqualTo(config.maxBucketCount());
 		assertThat(properties.getBaseTimeUnit()).isSameAs(config.baseTimeUnit());
 	}