docs: add basic docs for audit functionality.

README.md

@@ -145,6 +145,11 @@ To understand what's right for your organization, consider:
   - it's OK if your organization controls the creation/configuration of
     pipelines: this restricts the opportunity to misconfigure a pipeline.
 
+## Operations
+
+See the [observability documentation](./docs/observability.md) for more details
+on the information provided by the system when running.
+
 ## Configuration
 
 Requirements:

docs/observability.md

@@ -0,0 +1,58 @@
+# Observability
+
+The system produces traces and metrics via Open Telemetry, and logs via zerolog.
+There are minimal informational logs, as well as per-request audit logs that are
+written to the process's `stdout`.
+
+## Audit logs
+
+Audit logs provide a level of non-repudiation for the system. These logs are
+written to the container's stdout, and cannot be disabled.
+
+> [!TIP]
+> Requests to non-existent routes do not form part of the audit log. Access logs
+> or firewall logs are better sources for this information.
+
+Each authenticated endpoint (both `/token` and `/git-credentials`) will record
+details about the request, the authorization process, and the GitHub token
+created. If an error occurs, this is also written out.
+
+At a technical level, logs are written to stdout using zerolog at the "audit"
+log level. Initial data is collected by request middleware and the partial entry
+is then accessible via the context. The context entry is further enriched with
+details by other components, including both the JWT middleware and the vendor.
+such that the log is fully completed by the end of the request.
+
+> [!IMPORTANT]
+> A panic in the request chain will still result in the audit log being written,
+> and the panic details will also be included.
+
+### Audit log fields
+
+1. Request data
+    - `Method`: the request method. This will currently be `GET` for all standard requests.
+    - `Path`: the requested path.
+    - `Status` is the HTTP response status of the request
+    - `SourceIP` is the client IP of the requestor
+    - `UserAgent` is the user agent reported by the client
+    - `Error` is the error produced by the request. This may come from internal
+      errors or panics, as well as the JWT validation and token creation
+      components.
+2. Authorization data
+    - `Authorized` is a boolean that is `true` when the request JWT is
+      successfully authorized by the service.
+    - `AuthSubject` is the contents of the `sub` field from the JWT
+    - `AuthIssuer` is the JWT `iss` field
+    - `AuthAudience` is the (possibly multiple) reported `aud` field values from
+      the JWT
+    - `AuthExpirySecs` is the JWT expiry time in seconds after the Unix epoch
+3. Token data
+    - `Repositories` is the set of repositories that the token allows access to
+    - `Permissions` is the set of GitHub token permissions assigned to the token
+    - `ExpirySecs` is the GitHub token expiry time in seconds after the Unix
+      epoch
+
+## Open Telemetry
+
+This section is a stub. For now, refer to the [`.envrc`](../.envrc) file for
+details of all Open Telemetry related configuration that's currently possible.