feat: Add flagsmith_http_server_response_size_bytes metric

README.md

@@ -65,6 +65,7 @@ The following default metrics are exposed:
 - `flagsmith_build_info`: Has the labels `version` and `ci_commit_sha`.
 - `flagsmith_http_server_request_duration_seconds`: Histogram labeled with `method`, `route`, and `response_status`.
 - `flagsmith_http_server_requests_total`: Counter labeled with `method`, `route`, and `response_status`.
+- `flagsmith_http_server_response_size_bytes`:Histogram labeled with `method`, `route`, and `response_status`.
 - `flagsmith_task_processor_enqueued_tasks_total`: Counter labeled with `task_identifier`.
 
 ##### Task Processor metrics

src/common/gunicorn/constants.py

@@ -1 +1,12 @@
 WSGI_DJANGO_ROUTE_ENVIRON_KEY = "wsgi.django_route"
+HTTP_SERVER_RESPONSE_SIZE_DEFAULT_BUCKETS = (
+    # 1 kB, 10 kB, 100 kB, 500 kB, 1 MB, 5 MB, 10 MB
+    1 * 1024,
+    10 * 1024,
+    100 * 1024,
+    500 * 1024,
+    1 * 1024 * 1024,
+    5 * 1024 * 1024,
+    10 * 1024 * 1024,
+    float("inf"),
+)

src/common/gunicorn/logging.py

@@ -64,6 +64,9 @@ def access(
             duration_seconds
         )
         metrics.flagsmith_http_server_requests_total.labels(**labels).inc()
+        metrics.flagsmith_http_server_response_size_bytes.labels(**labels).observe(
+            resp.response_length or 0,
+        )
 
 
 class GunicornJsonCapableLogger(PrometheusGunicornLogger):

src/common/gunicorn/metrics.py

@@ -1,5 +1,7 @@
 import prometheus_client
+from django.conf import settings
 
+from common.gunicorn.constants import HTTP_SERVER_RESPONSE_SIZE_DEFAULT_BUCKETS
 from common.prometheus import Histogram
 
 flagsmith_http_server_requests_total = prometheus_client.Counter(
@@ -12,3 +14,13 @@
     "HTTP request duration in seconds",
     ["route", "method", "response_status"],
 )
+flagsmith_http_server_response_size_bytes = Histogram(
+    "flagsmith_http_server_response_size_bytes",
+    "HTTP response size in bytes",
+    ["route", "method", "response_status"],
+    buckets=getattr(
+        settings,
+        "PROMETHEUS_HTTP_SERVER_RESPONSE_SIZE_HISTOGRAM_BUCKETS",
+        HTTP_SERVER_RESPONSE_SIZE_DEFAULT_BUCKETS,
+    ),
+)

tests/unit/common/gunicorn/test_logging.py

@@ -1,5 +1,4 @@
 import logging
-import logging.config
 import os
 from datetime import datetime, timedelta
 
@@ -88,25 +87,31 @@ def test_gunicorn_prometheus_gunicorn_logger__expected_metrics(
     response_mock = mocker.Mock()
     response_mock.status = b"200 OK"
     response_mock.status_code = 200
+    response_mock.response_length = 42
 
     # When
     logger.access(
         response_mock,
         mocker.Mock(),
-        {"wsgi.django_route": "^health", "REQUEST_METHOD": "GET"},
+        {"wsgi.django_route": "/health", "REQUEST_METHOD": "GET"},
         timedelta(milliseconds=101),
     )
 
     # Then
     assert_metric(
         name="flagsmith_http_server_requests_total",
         value=1.0,
-        labels={"method": "GET", "route": "^health", "response_status": "200"},
+        labels={"method": "GET", "route": "/health", "response_status": "200"},
     )
     assert_metric(
         name="flagsmith_http_server_request_duration_seconds_sum",
         value=0.101,
-        labels={"method": "GET", "route": "^health", "response_status": "200"},
+        labels={"method": "GET", "route": "/health", "response_status": "200"},
+    )
+    assert_metric(
+        name="flagsmith_http_server_response_size_bytes_sum",
+        value=42.0,
+        labels={"method": "GET", "route": "/health", "response_status": "200"},
     )