Add option to configure quota project ID

Author: psx95

gcp-auth-extension/README.md

@@ -36,7 +36,10 @@ Here is a list of configurable options for the extension:
 
 - `GOOGLE_CLOUD_PROJECT`: Environment variable that represents the Google Cloud Project ID to which the telemetry needs to be exported.
   - Can also be configured using `google.cloud.project` system property.
-  - If this option is not configured, the extension would infer GCP Project ID from the application default credentials. For more information on application default credentials, see [here](https://cloud.google.com/docs/authentication/application-default-credentials).
+  - This is a required option, the telemetry export to GCP will fail.
+- `GOOGLE_CLOUD_QUOTA_PROJECT`: Environment variable that represents the Google Cloud Quota Project ID which will be charged for the GCP API usage. To learn more about a *quota project*, see [here](https://cloud.google.com/docs/quotas/quota-project).
+  - Can also be configured using `google.cloud.quota.project` system property.
+  - If this option is not configured, the extension would infer GCP Quota Project ID from the application default credentials. For more information on application default credentials, see [here](https://cloud.google.com/docs/authentication/application-default-credentials).
 
 ## Usage
 

gcp-auth-extension/src/main/java/io/opentelemetry/contrib/gcp/auth/ConfigurableOption.java

@@ -18,7 +18,18 @@ public enum ConfigurableOption {
    * Represents the Google Cloud Project ID option. Can be configured using the environment variable
    * `GOOGLE_CLOUD_PROJECT` or the system property `google.cloud.project`.
    */
-  GOOGLE_CLOUD_PROJECT("Google Cloud Project ID");
+  GOOGLE_CLOUD_PROJECT("Google Cloud Project ID"),
+
+  /**
+   * Represents the Google Cloud Quota Project ID option. Can be configured using the environment
+   * variable `GOOGLE_CLOUD_QUOTA_PROJECT` or the system property `google.cloud.quota.project`. The
+   * quota project is the project that is used for quota management and billing for the API usage.
+   *
+   * <p>The environment variable name is selected to be consistent with the <a
+   * href="https://cloud.google.com/docs/quotas/set-quota-project">official GCP client
+   * libraries</a>.
+   */
+  GOOGLE_CLOUD_QUOTA_PROJECT("Google Cloud Quota Project ID");
 
   private final String userReadableName;
   private final String environmentVariableName;