update_prometheus_rules hook restarts prometheus

We are using the prometheus2 charm with relations to some custom charms we have developed.

We have prometheus-client and prometheus-rules relations, however, we have noticed that the prometheus-rules relation hook is firing every 5 minutes in our custom charm.

Our custom charm runs this:

@when('prometheus-rules.available')
def render_prometheus_rules(prometheus_rules):
    # Send a list of rules for alerting to Prometheus
    formatted_rules = []
    template_files = glob.glob(os.path.join(get_templates_dir(), "rule_*.tmpl"))
    for template_file in template_files:
        with open(template_file, 'r') as fd:
            #formatted_rules.append(render_template(fd.read(), context))
            formatted_rules.append(fd.read())

    prometheus_rules.configure("\n".join(formatted_rules))

Then update_prometheus_rules fires within the prometheus2 charm:

@when('prometheus.started')
@when('prometheus-rules.available')
def update_prometheus_rules(prometheus_rules):
    rules = prometheus_rules.prometheus_rules()
    # insert into a template
    template = """
groups:
  - name: JujuRelation
    rules:
{{ rules }}
    """
    with open(PATHS['juju_rules_path'], 'w') as fh:
        fh.write(render_template(template, {'rules': "\n".join(rules)}))
    set_state('prometheus.do-check-reconfig')
    set_state('prometheus.do-restart')

The last line of this code forces a restart of Prometheus.

Therefore our Prometheus is restarting every 5 minutes even though the prometheus-rules aren't changing.

1. Is it correct that the prometheus-rules.available hook fires every 5 minutes? Shouldn't this just fire once?
2. Even if it does fire, why does it need to do a full Prometheus restart, isn't do-check-reconfig sufficient?

thanks,

Ed