feat: add backend middleware analytics for prometheus

  * Add custom metrics to estimate unique visitors.
  * Add custom metrics to estimate usefull client information (browser,
    os, device)
  * Add PRIVACY.md file

Signed-off-by: Alan Peixinho <alan.peixinho@profusion.mobi>

Author: alanpeixinho

PRIVACY.md

@@ -0,0 +1,81 @@
+# Privacy Policy
+
+This Privacy Policy explains what data the KernelCI Dashboard collects when you
+use it, why, and how we protect it. It is written for users of the service.
+Operators deploying the Dashboard are responsible for keeping this document
+accurate for their own instance and for surfacing it to their users.
+
+## Summary
+
+The KernelCI Dashboard collects **anonymous, aggregate usage analytics** only.
+We do **not** build user profiles, do **not** sell data, and do **not** store
+any personal identifier.
+
+## What we collect
+
+When you make requests to the Dashboard API (`/api/`), the backend records
+aggregate usage metrics. These are stored only as counts (Prometheus counters),
+never as per-user records or per-user timestamps:
+
+- **Request attributes**: the API endpoint name, HTTP method, response status
+  class (e.g. `2xx`), and coarse client buckets derived from your browser's
+  User-Agent (e.g. browser family `Chrome`, operating system `Linux`, device
+  type `desktop`). Automated clients are bucketed as `bot`.
+- **Referrer domain**: only the external domain that linked you to the
+  Dashboard (e.g. `example.org`). Same-site and direct visits are recorded as
+  `direct_or_internal`. The full referring URL is never stored.
+- **Unique visitor estimates**: daily de-duplicated visit counts, in total and
+  per endpoint.
+
+## What we do NOT collect or store
+
+- Your raw IP address.
+- Your raw, full User-Agent string.
+- The full referrer URL or any query parameters.
+- Any account, name, email, or other directly identifying information.
+- Any cross-day or long-term tracking identifier.
+
+## How unique visitors are counted
+
+To estimate unique visitors without identifying you, the backend:
+
+1. Computes a one-way fingerprint as
+   `HMAC-SHA256(daily_salt, "<your_ip>|<your_user_agent>")`.
+2. Uses a `daily_salt` — a random 32-byte secret generated fresh each UTC day,
+   held only in an in-memory cache (Redis) with a ~25 hour lifetime, never
+   written to disk.
+3. Uses only the resulting hash as a short-lived de-duplication key. Your raw
+   IP and User-Agent are discarded immediately after the hash is computed and
+   are never written to any metric or to disk.
+
+Because the salt is secret and rotates every day, fingerprints cannot be linked
+across days, and the hash cannot be reversed back to your IP/User-Agent in
+practice.
+
+## Legal basis and retention
+
+- **Purpose**: understanding aggregate usage and load to operate and improve
+  the service.
+- **Legal basis** (where GDPR applies): legitimate interest in operating and
+  improving the service, using the most privacy-preserving design we can.
+- **Retention**: the transient daily fingerprints and salt expire automatically
+  within ~25 hours. Only anonymous aggregate counters are retained beyond that;
+  these contain no personal data.
+
+## Your rights
+
+Because we retain only anonymous, aggregate counts and hold no identifier that
+can single you out beyond a single day, we cannot link any stored data to a
+specific person and therefore cannot act on individual access/deletion requests
+against the aggregate metrics. If you have questions about privacy on a specific
+deployment, contact that deployment's operator.
+
+## Changes
+
+We may update this policy as the service evolves. Changes are tracked in the
+repository's git history.
+
+## Technical reference
+
+For implementation details, see
+[`docs/monitoring.md`](docs/monitoring.md#client-analytics).

backend/kernelCI/settings.py

@@ -89,6 +89,7 @@ def get_json_env_var(name, default):
     "django.contrib.messages.middleware.MessageMiddleware",
     "django.middleware.clickjacking.XFrameOptionsMiddleware",
     "kernelCI_app.middleware.logServerErrorMiddleware.LogServerErrorMiddleware",
+    "kernelCI_app.middleware.backendRequestMetricsMiddleware.BackendRequestMetricsMiddleware",
     "django_prometheus.middleware.PrometheusAfterMiddleware",
 ]