fix: mark tracer initializer OSGi import optional

prometheus-metrics-core/pom.xml

@@ -69,4 +69,31 @@
       <scope>test</scope>
     </dependency>
   </dependencies>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.felix</groupId>
+        <artifactId>maven-bundle-plugin</artifactId>
+        <configuration>
+          <instructions>
+            <Automatic-Module-Name>${automatic.module.name}</Automatic-Module-Name>
+            <!-- Micrometer excludes this artifact and supplies its own supplier.
+                 The import must not be required for the bundle to resolve. -->
+            <Import-Package>
+              io.prometheus.metrics.tracer.initializer;resolution:=optional,*
+            </Import-Package>
+          </instructions>
+        </configuration>
+        <executions>
+          <execution>
+            <!-- Write MANIFEST.MF in process-classes for OSGi header tests. -->
+            <goals>
+              <goal>manifest</goal>
+            </goals>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
 </project>

prometheus-metrics-core/src/test/java/io/prometheus/metrics/core/exemplars/OsgiBundleManifestTest.java

@@ -0,0 +1,98 @@
+package io.prometheus.metrics.core.exemplars;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.InputStream;
+import java.net.URI;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.security.CodeSource;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.jar.JarFile;
+import java.util.jar.Manifest;
+import org.junit.jupiter.api.Test;
+
+class OsgiBundleManifestTest {
+
+  private static final String TRACER_INITIALIZER = "io.prometheus.metrics.tracer.initializer";
+
+  @Test
+  void tracerInitializerImportIsOptional() throws Exception {
+    Manifest manifest = loadBundleManifest(ExemplarSampler.class);
+    String header = manifest.getMainAttributes().getValue("Import-Package");
+    assertThat(header).as("Import-Package").isNotBlank();
+    String clause = requireClause(header, TRACER_INITIALIZER);
+    assertThat(clause)
+        .as("OSGi resolution of %s", TRACER_INITIALIZER)
+        .contains("resolution:=optional");
+  }
+
+  private static Manifest loadBundleManifest(Class<?> type) throws Exception {
+    CodeSource codeSource = type.getProtectionDomain().getCodeSource();
+    assertThat(codeSource).as("code source for %s", type.getName()).isNotNull();
+    URI location = codeSource.getLocation().toURI();
+    Path path = Path.of(location);
+    if (Files.isDirectory(path)) {
+      Path manifestFile = path.resolve("META-INF/MANIFEST.MF");
+      assertThat(Files.exists(manifestFile))
+          .as("bnd MANIFEST.MF for %s at %s", type.getName(), manifestFile)
+          .isTrue();
+      try (InputStream in = Files.newInputStream(manifestFile)) {
+        return new Manifest(in);
+      }
+    }
+    try (JarFile jar = new JarFile(path.toFile())) {
+      Manifest manifest = jar.getManifest();
+      assertThat(manifest).as("MANIFEST.MF in %s", path).isNotNull();
+      return manifest;
+    }
+  }
+
+  private static String requireClause(String header, String packageName) {
+    for (String rawClause : splitRespectingQuotes(header, ',')) {
+      String name = rawClause.trim();
+      int separator = indexOfUnquoted(name, ';');
+      if (separator >= 0) {
+        name = name.substring(0, separator).trim();
+      }
+      if (packageName.equals(name)) {
+        return rawClause.trim();
+      }
+    }
+    throw new AssertionError("missing Import-Package clause " + packageName);
+  }
+
+  private static int indexOfUnquoted(String value, char target) {
+    boolean inQuote = false;
+    for (int i = 0; i < value.length(); i++) {
+      char c = value.charAt(i);
+      if (c == '"') {
+        inQuote = !inQuote;
+      } else if (!inQuote && c == target) {
+        return i;
+      }
+    }
+    return -1;
+  }
+
+  private static List<String> splitRespectingQuotes(String value, char separator) {
+    List<String> parts = new ArrayList<>();
+    StringBuilder current = new StringBuilder();
+    boolean inQuote = false;
+    for (int i = 0; i < value.length(); i++) {
+      char c = value.charAt(i);
+      if (c == '"') {
+        inQuote = !inQuote;
+        current.append(c);
+      } else if (!inQuote && c == separator) {
+        parts.add(current.toString());
+        current.setLength(0);
+      } else {
+        current.append(c);
+      }
+    }
+    parts.add(current.toString());
+    return parts;
+  }
+}