doc: Add Fastly debug runbook

Author: Krinkle

doc/runbook-fastly-debug.md

@@ -0,0 +1,63 @@
+# Runbook: Fastly debugging
+
+## Real-time logging
+
+Aggregate statistics are available under the "Edge Observer" tab
+of the [Observability](https://docs.fastly.com/en/guides/about-the-observability-page)
+page in Fastly, this includes for example requests by cache response
+type (hit, miss, pass, synthetic, error), status code, object size,
+and HTTP version.
+
+
+There isn't a built-in way to review, e.g. a sampled web request log,
+but you can debug actual requests by setting up a logging endpoint
+under one of the "services" in your account. This endpiont then
+receives a stream of real-time log events.
+
+Documentation:
+* https://docs.fastly.com/en/guides/setting-up-remote-log-streaming
+* https://docs.fastly.com/en/guides/log-streaming-https
+* https://docs.fastly.com/en/guides/useful-conditions-for-logging
+* https://docs.fastly.com/en/guides/useful-variables-to-log
+
+### Example: Log "pass" requests
+
+* https://docs.fastly.com/en/guides/using-conditions
+* https://developer.fastly.com/reference/vcl/variables/
+* https://developer.fastly.com/reference/http/http-headers/
+
+Log the request URLs of requests that are considered uncachable
+(as opposed to a cache "miss").
+
+Condition:
+```
+req.http.Fastly-Cachetype == "PASS"
+```
+
+Logging endpoint:
+
+* Condition: "If pass"
+* Placement: "Format Version Default"
+* Log format: (simplified from the default to not contain any obvious PII)
+  ```json
+  {
+    "host": "%{if(req.http.Fastly-Orig-Host, req.http.Fastly-Orig-Host, req.http.Host)}V",
+    "url": "%{json.escape(req.url)}V",
+    "request_method": "%{json.escape(req.method)}V",
+    "response_status": %{resp.status}V
+  }
+  ```
+* URL: A newly generated URL from https://log-bin.fastly.dev/
+* Advanced
+  * Content type: `text/plain`
+  * Method: `POST`
+  * JSON log entry format: "Newline delimited"
+  * Select a log line format: "Blank"
+
+### Example: Sampled "miss" log
+
+* https://developer.fastly.com/reference/vcl/functions/randomness/randombool/
+
+```
+req.http.Fastly-Cachetype == "PASS"
+```