Provide methods to support fallback value for config options

Author: psx95

javaagent-extensions/gcp-auth/src/main/java/com/google/cloud/opentelemetry/extension/auth/ConfigurableOption.java

@@ -17,6 +17,7 @@
 
 import io.opentelemetry.sdk.autoconfigure.spi.ConfigurationException;
 import java.util.Locale;
+import java.util.function.Supplier;
 
 /**
  * An enum representing configurable options for a GCP Authentication Extension. Each option has a
@@ -65,7 +66,7 @@ String getSystemProperty() {
    * Retrieves the configured value for this option. This method checks the environment variable
    * first and then the system property.
    *
-   * @return the configured value as a string, or throws an exception if not configured.
+   * @return The configured value as a string, or throws an exception if not configured.
    * @throws ConfigurationException if neither the environment variable nor the system property is
    *     set.
    */
@@ -86,4 +87,22 @@ String getConfiguredValue() throws ConfigurationException {
               this.getSystemProperty()));
     }
   }
+
+  /**
+   * Retrieves the value for this option, prioritizing environment variables and system properties.
+   * If neither an environment variable nor a system property is set for this option, the provided
+   * fallback function is used to determine the value.
+   *
+   * @param fallback A {@link Supplier} that provides the default value for the option when it is
+   *     not explicitly configured via an environment variable or system property.
+   * @return The configured value for the option, obtained from the environment variable, system
+   *     property, or the fallback function, in that order of precedence.
+   */
+  String getConfiguredValueWithFallback(Supplier<String> fallback) {
+    try {
+      return this.getConfiguredValue();
+    } catch (ConfigurationException e) {
+      return fallback.get();
+    }
+  }
 }

javaagent-extensions/gcp-auth/src/main/java/com/google/cloud/opentelemetry/extension/auth/GcpAuthAutoConfigurationCustomizerProvider.java

@@ -117,11 +117,20 @@ private Map<String, String> getRequiredHeaderMap(GoogleCredentials credentials)
 
   // Updates the current resource with the attributes required for ingesting OTLP data on GCP.
   private Resource customizeResource(Resource resource, ConfigProperties configProperties) {
+    String gcpProjectId =
+        ConfigurableOption.GOOGLE_CLOUD_PROJECT.getConfiguredValueWithFallback(
+            () -> {
+              try {
+                GoogleCredentials googleCredentials = GoogleCredentials.getApplicationDefault();
+                return googleCredentials.getQuotaProjectId();
+              } catch (IOException e) {
+                throw new GoogleAuthException(Reason.FAILED_ADC_RETRIEVAL, e);
+              }
+            });
+
     Resource res =
         Resource.create(
-            Attributes.of(
-                AttributeKey.stringKey(GCP_USER_PROJECT_ID_KEY),
-                ConfigurableOption.GOOGLE_CLOUD_PROJECT.getConfiguredValue()));
+            Attributes.of(AttributeKey.stringKey(GCP_USER_PROJECT_ID_KEY), gcpProjectId));
     return resource.merge(res);
   }
 }