@UnstableApi
    public AbstractCircuitBreakerClientBuilder<I, O> mapping(CircuitBreakerMapping mapping) {
        handler = CircuitBreakerClientHandler.of(requireNonNull(mapping, "mapping"));
        return this;
    }
Sets the {@link CircuitBreakerMapping}. If unspecified, {@link CircuitBreakerMapping#ofDefault()}
will be used. Note that the {@link CircuitBreakerClientHandler} set by calling
{@link #handler(CircuitBreakerClientHandler)} will be overwritten by calling this method.

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