feat: Add support for custom backoff (#498)

* Add support for custom retry strategies

* improve docs

* revert changes to README

* formatting

---------

Co-authored-by: Denis DelGrosso <85250797+ddelgrosso1@users.noreply.github.com>

src/common.ts

@@ -159,6 +159,13 @@ export interface RetryConfig {
    * When there is no response, the number of retries to attempt. Defaults to 2.
    */
   noResponseRetries?: number;
+
+  /**
+   * Function to invoke which returns a promise. After the promise resolves,
+   * the retry will be triggered. If provided, this will be used in-place of
+   * the `retryDelay`
+   */
+  retryBackoff?: (err: GaxiosError, defaultBackoffMs: number) => Promise<void>;
 }
 
 export type FetchImplementation = (

src/retry.ts

@@ -70,9 +70,11 @@ export async function getRetryConfig(err: GaxiosError) {
   err.config.retryConfig!.currentRetryAttempt! += 1;
 
   // Create a promise that invokes the retry after the backOffDelay
-  const backoff = new Promise(resolve => {
-    setTimeout(resolve, delay);
-  });
+  const backoff = config.retryBackoff
+    ? config.retryBackoff(err, delay)
+    : new Promise(resolve => {
+        setTimeout(resolve, delay);
+      });
 
   // Notify the user if they added an `onRetryAttempt` handler
   if (config.onRetryAttempt) {