NGINX Canary & Blue-Green Routing Prompt

You are a senior release engineer who ships canary and blue-green deployments through NGINX without a service mesh. You care about cohort stickiness, instant rollback, and being able to test the canary yourself before real traffic sees it.

I will provide:
- My two backend pools (stable and canary), as host:port lists: [PASTE UPSTREAMS]
- The routing strategy I want (percentage canary, or blue-green cutover): [DESCRIBE]
- The starting canary percentage and how I plan to ramp it: [e.g. 1% → 5% → 25% → 100%]
- Whether sessions must be sticky, and what stable key I can hash on (cookie, IP): [DESCRIBE]
- How I want to force MYSELF onto the canary for testing (header, cookie, source IP): [DESCRIBE]

Design the routing:

1. **split_clients cohort** — a `split_clients "${key}" $variant { N% canary; * stable; }` block. Explain why the key must be stable (so a user doesn't flip between variants) and what happens as I change the percentage.

2. **Upstreams** — define `upstream stable` and `upstream canary` (or a single upstream with `weight=` for the simplest case), and map `$variant` to the chosen `proxy_pass` target with a `map`.

3. **Override for testers** — a way to bypass the percentage split entirely (e.g. `X-Canary: 1` header or a cookie) so I can hit the canary deterministically, plus a way to pin known-good traffic to stable.

4. **Blue-green variant** — show the one-line change (a `map` default or a variable) that flips 100% of traffic between blue and green, and the reverse for rollback.

5. **Observability** — add the variant to the access log (`$variant` in `log_format`) and set a response header (`X-Served-Pool`) so I can confirm which pool served a request.

Output: (a) the full `http`-level `split_clients`/`map`/`upstream` config plus the `location` block, commented; (b) the exact edit-and-reload steps to ramp the percentage; (c) the single change that triggers rollback; (d) `curl` commands proving the header override and the pool header work. Apply only after `nginx -t` passes and reload.