Rename get_usage_for_all_services to get_data_for_billing_report

That describes what it does more correctly. As it does not include
services with no usage.

app/platform_stats/rest.py

@@ -59,7 +59,8 @@ def validate_date_range_is_within_a_financial_year(start_date, end_date):
 
 
 @platform_stats_blueprint.route('usage-for-all-services')
-def get_usage_for_all_services():
+@platform_stats_blueprint.route('data-for-billing-report')
+def get_data_for_billing_report():
     start_date = request.args.get('start_date')
     end_date = request.args.get('end_date')
 

tests/app/platform_stats/test_rest.py

@@ -126,11 +126,15 @@ def test_validate_date_is_within_a_financial_year_when_input_is_not_a_date(start
     assert e.value.status_code == 400
 
 
-def test_get_usage_for_all_services(notify_db_session, admin_request):
+def test_get_data_for_billing_report(notify_db_session, admin_request):
     setup = set_up_usage_data(datetime(2019, 5, 1))
-    response = admin_request.get("platform_stats.get_usage_for_all_services",
+    response = admin_request.get(
+        "platform_stats.get_data_for_billing_report",
         start_date='2019-05-01',
-                                 end_date='2019-06-30')
+        end_date='2019-06-30'
+    )
+
+    # we set up 5 services, but only 4 returned. service_with_emails was skipped as it had no bills to pay
     assert len(response) == 4
     assert response[0]["organisation_id"] == str(setup["org_1"].id)
     assert response[0]["service_id"] == str(setup["service_1_sms_and_letter"].id)