Add support for custom retry delays from onRetry callback

Allow onRetry callback to return a number to specify a custom delay for
the next retry attempt. This enables implementing Retry-After header
support and other dynamic retry delay strategies.

Author: jdalton

src/promises.ts

@@ -23,7 +23,7 @@ export interface RetryOptions {
     attempt: number,
     error: unknown,
     delay: number,
-  ) => boolean | undefined
+  ) => boolean | number | undefined
   onRetryCancelOnFalse?: boolean
   onRetryRethrow?: boolean
   retries?: number
@@ -323,6 +323,10 @@ export async function pRetry<T>(
           if (result === false && onRetryCancelOnFalse) {
             break
           }
+          // If onRetry returns a number, use it as the custom delay.
+          if (typeof result === 'number' && result >= 0) {
+            waitTime = Math.min(result, maxDelayMs as number)
+          }
         } catch (e) {
           if (onRetryRethrow) {
             throw e