Change default Prometheus host to localhost (#8298)

Co-authored-by: Jack Berg <34418638+jack-berg@users.noreply.github.com>

Author: utafrali

exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus/PrometheusHttpServerBuilder.java

@@ -18,16 +18,21 @@
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.function.Predicate;
+import java.util.logging.Logger;
 import javax.annotation.Nullable;
 
 /** A builder for {@link PrometheusHttpServer}. */
 public final class PrometheusHttpServerBuilder {
 
   static final int DEFAULT_PORT = 9464;
-  private static final String DEFAULT_HOST = "0.0.0.0";
+  private static final String DEFAULT_HOST = "localhost";
   private static final MemoryMode DEFAULT_MEMORY_MODE = MemoryMode.REUSABLE_DATA;
+  private static final Logger LOGGER =
+      Logger.getLogger(PrometheusHttpServerBuilder.class.getName());
 
-  private String host = DEFAULT_HOST;
+  // Temporarily nullable to detect when it's not set and log warning about 0.0.0.0 -> localhost
+  // change
+  @Nullable private String host;
   private int port = DEFAULT_PORT;
   private PrometheusRegistry prometheusRegistry = new PrometheusRegistry();
   private PrometheusMetricReaderBuilder metricReaderBuilder = PrometheusMetricReader.builder();
@@ -51,7 +56,12 @@ public final class PrometheusHttpServerBuilder {
     this.authenticator = builder.authenticator;
   }
 
-  /** Sets the host to bind to. If unset, defaults to {@value #DEFAULT_HOST}. */
+  /**
+   * Sets the host to bind to. If unset, defaults to {@value #DEFAULT_HOST}.
+   *
+   * <p>Previously defaulted to {@code 0.0.0.0}. To restore the old behavior, set host to {@code
+   * 0.0.0.0} explicitly.
+   */
   public PrometheusHttpServerBuilder setHost(String host) {
     requireNonNull(host, "host");
     checkArgument(!host.isEmpty(), "host must not be empty");
@@ -173,9 +183,17 @@ public PrometheusHttpServer build() {
           "MemoryMode REUSEABLE_DATA cannot be used with custom executor, "
               + "since data may be corrupted if reading metrics concurrently");
     }
+    String resolvedHost = host;
+    if (resolvedHost == null) {
+      // TODO (jack-berg): Remove log after 1.64.0 release
+      LOGGER.info(
+          "PrometheusHttpServer host not set, defaulting to localhost. Previously defaulted to 0.0.0.0. "
+              + "If you depend on the old behavior, set host to 0.0.0.0 explicitly.");
+      resolvedHost = DEFAULT_HOST;
+    }
     return new PrometheusHttpServer(
         new PrometheusHttpServerBuilder(this), // copy to prevent modification
-        host,
+        resolvedHost,
         port,
         executor,
         prometheusRegistry,

exporters/prometheus/src/test/java/io/opentelemetry/exporter/prometheus/internal/PrometheusMetricReaderProviderTest.java

@@ -59,7 +59,8 @@ void createMetricReader_Default() throws IOException {
           .extracting("server", as(InstanceOfAssertFactories.type(HttpServer.class)))
           .satisfies(
               server -> {
-                assertThat(server.getAddress().getAddress().isAnyLocalAddress()).isTrue();
+                assertThat(server.getAddress().getHostName())
+                    .isIn("localhost", "127.0.0.1", "kubernetes.docker.internal");
                 assertThat(server.getAddress().getPort()).isEqualTo(9464);
               });
       assertThat(metricReader.getMemoryMode()).isEqualTo(MemoryMode.REUSABLE_DATA);