@UnstableApi
    protected final String toString(Consumer<? super StringBuilder> builderMutator) {
        final StringBuilder buf = new StringBuilder();
        buf.append(getClass().getSimpleName());
        buf.append("{selector=").append(toStringSelector());
        buf.append(", allowsEmptyEndpoints=").append(allowEmptyEndpoints);
        buf.append(", initialized=").append(initialEndpointsFuture.isDone());
        buf.append(", numEndpoints=").append(endpoints.size());
        buf.append(", endpoints=").append(truncate(endpoints, 10));
        builderMutator.accept(buf);
        return buf.append('}').toString();
    }
Returns the string representation of this {@link DynamicEndpointGroup}. Specify a {@link Consumer}
to add more fields to the returned string, e.g.
<pre>{@code
> @Override
> public String toString() {
>     return toString(buf -> {
>         buf.append(", foo=").append(foo);
>         buf.append(", bar=").append(bar);
>     });
> }
}</pre>

@param builderMutator the {@link Consumer} that appends the additional fields into the given
                      {@link StringBuilder}.
core/src/main/java/com/linecorp/armeria/client/endpoint/DynamicEndpointGroup.java