Added Prometheus metrics support with opt-in CLI flag.

tests/test_metrics.py

@@ -0,0 +1,113 @@
+# Copyright 2026 Logan Fick
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Unit tests for the Prometheus metrics module.
+
+Tests cover metric object registration and the MetricsServer HTTP endpoint.
+"""
+
+from typing import TYPE_CHECKING
+
+import aiohttp
+import pytest
+from prometheus_client import generate_latest
+
+from crabstero.metrics import MetricsServer
+
+if TYPE_CHECKING:
+    from collections.abc import AsyncGenerator
+
+
+class TestMetricObjects:
+    """All expected metric objects exist and are registered."""
+
+    @pytest.mark.parametrize(
+        "name",
+        [
+            pytest.param("crabstero_build_info", id="build-info"),
+            pytest.param("crabstero_messages_ingested_total", id="messages-ingested"),
+            pytest.param("crabstero_replies_sent_total", id="replies-sent"),
+            pytest.param("crabstero_embeds_generated_total", id="embeds-generated"),
+            pytest.param("crabstero_messages_processed_total", id="messages-processed"),
+            pytest.param("crabstero_channels_ingested_total", id="channels-ingested"),
+            pytest.param(
+                "crabstero_channel_ingestion_errors_total",
+                id="channel-ingestion-errors",
+            ),
+            pytest.param("crabstero_pingme_opted_in_total", id="pingme-opted-in"),
+            pytest.param("crabstero_pingme_opted_out_total", id="pingme-opted-out"),
+            pytest.param("crabstero_pingme_errors_total", id="pingme-errors"),
+            pytest.param("crabstero_discord_latency_seconds", id="discord-latency"),
+            pytest.param("crabstero_guild_count", id="guild-count"),
+            pytest.param(
+                "crabstero_ingestion_backlog_channels", id="ingestion-backlog"
+            ),
+            pytest.param(
+                "crabstero_message_ingestion_duration_seconds",
+                id="message-ingestion-duration",
+            ),
+            pytest.param(
+                "crabstero_channel_ingestion_duration_seconds",
+                id="channel-ingestion-duration",
+            ),
+            pytest.param(
+                "crabstero_generation_duration_seconds", id="generation-duration"
+            ),
+            pytest.param(
+                "crabstero_generated_message_length_characters",
+                id="generated-message-length",
+            ),
+            pytest.param(
+                "crabstero_channel_ingestion_messages", id="channel-ingestion-messages"
+            ),
+            pytest.param("crabstero_discord_events_total", id="discord-events"),
+        ],
+    )
+    def test_metric_in_output(self, name: str) -> None:
+        """Each declared metric appears in the Prometheus text output."""
+        output = generate_latest().decode()
+        assert name in output, f"{name} not found in Prometheus output"
+
+
+@pytest.fixture
+async def metrics_server() -> AsyncGenerator[MetricsServer]:
+    """Start a MetricsServer on an OS-assigned port and stop it after the test."""
+    server = MetricsServer("127.0.0.1", 0)
+    await server.start()
+    yield server
+    await server.stop()
+
+
+class TestMetricsServer:
+    """HTTP server serves Prometheus metrics on /metrics."""
+
+    async def test_serves_metrics_endpoint(self, metrics_server: MetricsServer) -> None:
+        """GET /metrics returns 200 with metric output containing our metrics."""
+        async with (
+            aiohttp.ClientSession() as session,
+            session.get(f"http://127.0.0.1:{metrics_server.port}/metrics") as resp,
+        ):
+            assert resp.status == 200
+            body = await resp.text()
+            assert "crabstero_build_info" in body
+
+    async def test_non_metrics_path_returns_404(
+        self, metrics_server: MetricsServer
+    ) -> None:
+        """GET on an unknown path returns 404."""
+        async with (
+            aiohttp.ClientSession() as session,
+            session.get(f"http://127.0.0.1:{metrics_server.port}/notfound") as resp,
+        ):
+            assert resp.status == 404