final CircuitBreakerClientHandler handler() {
        return handler;
    }
Sets the {@link CircuitBreakerClientHandler}. Note that the {@link CircuitBreakerMapping}
set by calling {@link #mapping(CircuitBreakerMapping)} 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