bootstrap and django keep button and input on same line

I would like to be my select input and button on the same line. This i can get to work, but my input is stretched to the col width and not the input width.

How can i achieve this?

my code so far:

<form action="{% url 'register_user' %}" method="post">
    {% csrf_token %}

    <div class="row">
    {% for field in filter_form %}
        <div class="col-md-9">
            <div class="form-group">
                {{ field|addcss:"form-control" }}
            </div>
        </div>
        <div class="col-md-3">
            <button type="submit" class="btn btn-primary btn-sm">Filter</button>
        </div>
    {% endfor %}
    </div>
</form>

What am i missing?

Side note: is there some way to make my button a bit wider than the text in it?