Share some metrics between the Prometheus exporter and the phone home stats (#13671)

tests/test_phone_home.py

@@ -15,8 +15,14 @@
 import resource
 from unittest import mock
 
+from twisted.test.proto_helpers import MemoryReactor
+
 from synapse.app.phone_stats_home import phone_stats_home
+from synapse.rest import admin
+from synapse.rest.client import login, sync
+from synapse.server import HomeServer
 from synapse.types import JsonDict
+from synapse.util import Clock
 
 from tests.unittest import HomeserverTestCase
 
@@ -47,5 +53,43 @@ class PhoneHomeStatsTestCase(HomeserverTestCase):
         stats: JsonDict = {}
         self.reactor.advance(1)
         # `old_resource` has type `Mock` instead of `struct_rusage`
-        self.get_success(phone_stats_home(self.hs, stats, past_stats))  # type: ignore[arg-type]
+        self.get_success(
+            phone_stats_home(self.hs, stats, past_stats)  # type: ignore[arg-type]
+        )
         self.assertApproximates(stats["cpu_average"], 100, tolerance=2.5)
+
+
+class CommonMetricsTestCase(HomeserverTestCase):
+    servlets = [
+        admin.register_servlets,
+        login.register_servlets,
+        sync.register_servlets,
+    ]
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.metrics_manager = hs.get_common_usage_metrics_manager()
+        self.get_success(self.metrics_manager.setup())
+
+    def test_dau(self) -> None:
+        """Tests that the daily active users count is correctly updated."""
+        self._assert_metric_value("daily_active_users", 0)
+
+        self.register_user("user", "password")
+        tok = self.login("user", "password")
+        self.make_request("GET", "/sync", access_token=tok)
+
+        self.pump(1)
+
+        self._assert_metric_value("daily_active_users", 1)
+
+    def _assert_metric_value(self, metric_name: str, expected: int) -> None:
+        """Compare the given value to the current value of the common usage metric with
+        the given name.
+
+        Args:
+            metric_name: The metric to look up.
+            expected: Expected value for this metric.
+        """
+        metrics = self.get_success(self.metrics_manager.get_metrics())
+        value = getattr(metrics, metric_name)
+        self.assertEqual(value, expected)