check request body against regex

tests/test_charm.py

@@ -63,7 +63,7 @@ def test_metrics_endpoint_relation_created(self, open):
         mock_opener.expect(
             url='http://localhost/metrics-users',
             method='POST',
-            # TODO: test body
+            body_regex=r'{"username": "juju-metrics-r\d+", "password": ".*"}',
             response=mockResponse(
                 headers=mockHeaders(content_type='application/json'),
                 body='{"message":"created user \\"juju-metrics-r0\\""}'
@@ -88,15 +88,16 @@ class mockOpener:
     def __init__(self, test_case):
         self.test = test_case
 
-    def expect(self, url, method, response):
+    def expect(self, url, method, body_regex, response):
         self.url = url
         self.method = method
-
+        self.body_regex = body_regex
         self.response = response
 
     def open(self, request, timeout):
         self.test.assertEqual(request.full_url, self.url)
         self.test.assertEqual(request.method, self.method)
+        self.test.assertRegex(str(request.data), expected_regex=self.body_regex)
 
         return self.response