@UnstableApi
    public AbstractCircuitBreakerClientBuilder<I, O> recover(
            BiFunction<? super ClientRequestContext, ? super I, ? extends O> fallback) {
        requireNonNull(fallback, "fallback");
        this.fallback = fallback;
        return this;
    }
Sets the {@link BiFunction}. This is invoked when adding the fallback strategy.
<p>For example:</p>
<pre>{@code
CircuitBreakerClient
  .builder(...)
  .recover((ctx, req) -> {
      // fallback logic
      return HttpResponse.of(...);
  });
}</pre>

@return {@code this} to support method chaining.
core/src/main/java/com/linecorp/armeria/client/circuitbreaker/AbstractCircuitBreakerClientBuilder.java