Implement prometheus exporter provider

exporters/prometheus/build.gradle.kts

@@ -11,6 +11,8 @@ otelJava.moduleName.set("io.opentelemetry.exporter.prometheus")
 dependencies {
   api(project(":sdk:metrics"))
 
+  implementation(project(":sdk-extensions:autoconfigure-spi"))
+
   compileOnly("com.sun.net.httpserver:http")
 
   testImplementation("com.google.guava:guava")

exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus/internal/PrometheusCustomizerProvider.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.exporter.prometheus.internal;
+
+import io.opentelemetry.exporter.prometheus.PrometheusHttpServer;
+import io.opentelemetry.exporter.prometheus.PrometheusHttpServerBuilder;
+import io.opentelemetry.sdk.autoconfigure.spi.AutoConfigurationCustomizer;
+import io.opentelemetry.sdk.autoconfigure.spi.AutoConfigurationCustomizerProvider;
+import io.opentelemetry.sdk.autoconfigure.spi.ConfigProperties;
+
+/**
+ * SPI implementation for {@link PrometheusHttpServer}.
+ *
+ * <p>This class is internal and is hence not for public use. Its APIs are unstable and can change
+ * at any time.
+ */
+public class PrometheusCustomizerProvider implements AutoConfigurationCustomizerProvider {
+
+  @Override
+  public void customize(AutoConfigurationCustomizer autoConfiguration) {
+    autoConfiguration.addMeterProviderCustomizer(
+        (builder, config) -> {
+          boolean prometheusEnabled =
+              config.getList("otel.metrics.exporter").contains("prometheus");
+          if (prometheusEnabled) {
+            builder.registerMetricReader(configurePrometheusHttpServer(config));
+          }
+          return builder;
+        });
+  }
+
+  private static PrometheusHttpServer configurePrometheusHttpServer(ConfigProperties config) {
+    PrometheusHttpServerBuilder prometheusBuilder = PrometheusHttpServer.builder();
+
+    Integer port = config.getInt("otel.exporter.prometheus.port");
+    if (port != null) {
+      prometheusBuilder.setPort(port);
+    }
+    String host = config.getString("otel.exporter.prometheus.host");
+    if (host != null) {
+      prometheusBuilder.setHost(host);
+    }
+    return prometheusBuilder.build();
+  }
+}

exporters/prometheus/src/main/resources/META-INF/services/io.opentelemetry.sdk.autoconfigure.spi.AutoConfigurationCustomizerProvider

@@ -0,0 +1 @@
+io.opentelemetry.exporter.prometheus.internal.PrometheusCustomizerProvider

sdk-extensions/autoconfigure/build.gradle.kts

@@ -14,8 +14,6 @@ dependencies {
 
   implementation(project(":semconv"))
 
-  compileOnly(project(":exporters:prometheus"))
-
   annotationProcessor("com.google.auto.value:auto-value")
 
   testImplementation(project(":sdk:trace-shaded-deps"))

sdk-extensions/autoconfigure/src/main/java/io/opentelemetry/sdk/autoconfigure/ClasspathUtil.java

@@ -9,11 +9,18 @@
 
 final class ClasspathUtil {
 
-  @SuppressWarnings("UnusedException")
-  static void checkClassExists(String className, String featureName, String requiredLibrary) {
+  static boolean checkClassExists(String className) {
     try {
       Class.forName(className);
+      return true;
     } catch (ClassNotFoundException unused) {
+      return false;
+    }
+  }
+
+  @SuppressWarnings("UnusedException")
+  static void checkClassExists(String className, String featureName, String requiredLibrary) {
+    if (!checkClassExists(className)) {
       throw new ConfigurationException(
           featureName
               + " enabled but "

sdk-extensions/autoconfigure/src/main/java/io/opentelemetry/sdk/autoconfigure/MeterProviderConfiguration.java

@@ -15,6 +15,7 @@
 import io.opentelemetry.sdk.metrics.internal.exemplar.ExemplarFilter;
 import java.util.Collections;
 import java.util.List;
+import java.util.Objects;
 import java.util.Set;
 import java.util.function.BiFunction;
 import java.util.stream.Collectors;
@@ -70,6 +71,7 @@ static List<MetricReader> configureMetricReaders(
             exporterName ->
                 MetricExporterConfiguration.configureReader(
                     exporterName, config, serviceClassLoader, metricExporterCustomizer))
+        .filter(Objects::nonNull)
         .collect(Collectors.toList());
   }
 

sdk-extensions/autoconfigure/src/main/java/io/opentelemetry/sdk/autoconfigure/MetricExporterConfiguration.java

@@ -5,8 +5,6 @@
 
 package io.opentelemetry.sdk.autoconfigure;
 
-import io.opentelemetry.exporter.prometheus.PrometheusHttpServer;
-import io.opentelemetry.exporter.prometheus.PrometheusHttpServerBuilder;
 import io.opentelemetry.sdk.autoconfigure.spi.ConfigProperties;
 import io.opentelemetry.sdk.autoconfigure.spi.ConfigurationException;
 import io.opentelemetry.sdk.autoconfigure.spi.metrics.ConfigurableMetricExporterProvider;
@@ -17,6 +15,7 @@
 import java.util.HashMap;
 import java.util.Map;
 import java.util.function.BiFunction;
+import javax.annotation.Nullable;
 
 final class MetricExporterConfiguration {
 
@@ -30,22 +29,34 @@ final class MetricExporterConfiguration {
     EXPORTER_ARTIFACT_ID_BY_NAME.put("otlp", "opentelemetry-exporter-otlp");
   }
 
+  @Nullable
   static MetricReader configureReader(
       String name,
       ConfigProperties config,
       ClassLoader serviceClassLoader,
       BiFunction<? super MetricExporter, ConfigProperties, ? extends MetricExporter>
           metricExporterCustomizer) {
     if (name.equals("prometheus")) {
-      return configurePrometheusMetricReader(config);
+      if (!ClasspathUtil.checkClassExists(
+          "io.opentelemetry.exporter.prometheus.PrometheusHttpServer")) {
+        // PrometheusHttpServer is implemented as MetricReader (not MetricExporter) and uses
+        // the AutoConfigurationCustomizer#addMeterProviderCustomizer SPI hook instead of
+        // ConfigurableMetricExporterProvider. While the prometheus SPI hook is not handled here,
+        // the classpath check here provides uniform exception messages.
+        throw missingExporterException("prometheus", "opentelemetry-exporter-prometheus");
+      }
+      return null;
     }
 
     NamedSpiManager<MetricExporter> spiExportersManager =
         metricExporterSpiManager(config, serviceClassLoader);
 
     MetricExporter metricExporter = configureExporter(name, spiExportersManager);
     metricExporter = metricExporterCustomizer.apply(metricExporter, config);
-    return configurePeriodicMetricReader(config, metricExporter);
+
+    return PeriodicMetricReader.builder(metricExporter)
+        .setInterval(config.getDuration("otel.metric.export.interval", DEFAULT_EXPORT_INTERVAL))
+        .build();
   }
 
   // Visible for testing
@@ -66,42 +77,21 @@ static MetricExporter configureExporter(
     if (metricExporter == null) {
       String artifactId = EXPORTER_ARTIFACT_ID_BY_NAME.get(name);
       if (artifactId != null) {
-        throw new ConfigurationException(
-            "otel.metrics.exporter set to \""
-                + name
-                + "\" but "
-                + artifactId
-                + " not found on classpath. Make sure to add it as a dependency.");
+        throw missingExporterException(name, artifactId);
       }
       throw new ConfigurationException("Unrecognized value for otel.metrics.exporter: " + name);
     }
     return metricExporter;
   }
 
-  private static PeriodicMetricReader configurePeriodicMetricReader(
-      ConfigProperties config, MetricExporter exporter) {
-
-    return PeriodicMetricReader.builder(exporter)
-        .setInterval(config.getDuration("otel.metric.export.interval", DEFAULT_EXPORT_INTERVAL))
-        .build();
-  }
-
-  private static PrometheusHttpServer configurePrometheusMetricReader(ConfigProperties config) {
-    ClasspathUtil.checkClassExists(
-        "io.opentelemetry.exporter.prometheus.PrometheusHttpServer",
-        "Prometheus Metrics Server",
-        "opentelemetry-exporter-prometheus");
-    PrometheusHttpServerBuilder prom = PrometheusHttpServer.builder();
-
-    Integer port = config.getInt("otel.exporter.prometheus.port");
-    if (port != null) {
-      prom.setPort(port);
-    }
-    String host = config.getString("otel.exporter.prometheus.host");
-    if (host != null) {
-      prom.setHost(host);
-    }
-    return prom.build();
+  private static ConfigurationException missingExporterException(
+      String exporterName, String artifactId) {
+    return new ConfigurationException(
+        "otel.metrics.exporter set to \""
+            + exporterName
+            + "\" but "
+            + artifactId
+            + " not found on classpath. Make sure to add it as a dependency.");
   }
 
   private MetricExporterConfiguration() {}

sdk-extensions/autoconfigure/src/test/java/io/opentelemetry/sdk/autoconfigure/NotOnClasspathTest.java

@@ -153,8 +153,8 @@ void prometheus() {
                     (a, unused) -> a))
         .isInstanceOf(ConfigurationException.class)
         .hasMessageContaining(
-            "Prometheus Metrics Server enabled but opentelemetry-exporter-prometheus not found on "
-                + "classpath");
+            "otel.metrics.exporter set to \"prometheus\" but opentelemetry-exporter-prometheus not found on classpath."
+                + " Make sure to add it as a dependency.");
   }
 
   @Test

sdk-extensions/autoconfigure/src/testFullConfig/java/io/opentelemetry/sdk/autoconfigure/ConfigurableMetricExporterTest.java

@@ -10,7 +10,6 @@
 
 import com.google.common.collect.ImmutableMap;
 import io.opentelemetry.exporter.otlp.metrics.OtlpGrpcMetricExporter;
-import io.opentelemetry.exporter.prometheus.PrometheusHttpServer;
 import io.opentelemetry.sdk.autoconfigure.spi.ConfigProperties;
 import io.opentelemetry.sdk.autoconfigure.spi.ConfigurationException;
 import io.opentelemetry.sdk.autoconfigure.spi.internal.DefaultConfigProperties;
@@ -105,7 +104,7 @@ void defaultExporter() {
   void configureMultipleMetricExporters() {
     ConfigProperties config =
         DefaultConfigProperties.createForTest(
-            ImmutableMap.of("otel.metrics.exporter", "otlp,prometheus"));
+            ImmutableMap.of("otel.metrics.exporter", "otlp,logging"));
 
     assertThat(
             MeterProviderConfiguration.configureMetricReaders(
@@ -114,7 +113,7 @@ void configureMultipleMetricExporters() {
                 (metricExporter, unused) -> metricExporter))
         .hasSize(2)
         .hasAtLeastOneElementOfType(PeriodicMetricReader.class)
-        .hasAtLeastOneElementOfType(PrometheusHttpServer.class)
+        .hasAtLeastOneElementOfType(PeriodicMetricReader.class)
         .allSatisfy(metricReader -> metricReader.shutdown().join(10, TimeUnit.SECONDS));
   }
 }