feat(flags): return real feature flag evaluation for basic matching with new endpoint

[dmarticus]

The new Rust /flags endpoint now works end-to-end locally! With this change, you can now do the following:

• start the Rust service with RUST_LOG=debug cargo run --bin feature-flags
• call the endpoint with /decide-looking requests, e.g.

$ curl 'http://localhost:3001/flags?v=3&ip=1&_=1723503992996&ver=1.154.3' \
  -H 'Accept: */*' \
  -H 'Accept-Language: en-US,en;q=0.9,es;q=0.8,fi;q=0.7,sv;q=0.6' \
  -H 'Connection: keep-alive' \
  -H 'Content-Type: application/json' \
  -H 'Origin: https://localhost:3000' \
  -H 'Sec-Fetch-Dest: empty' \
  -H 'Sec-Fetch-Mode: cors' \
  -H 'Sec-Fetch-Site: cross-site' \
  -H 'User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/127.0.0.0 Safari/537.36' \
  -H 'sec-ch-ua: "Not)A;Brand";v="99", "Google Chrome";v="127", "Chromium";v="127"' \
  -H 'sec-ch-ua-mobile: ?0' \
  -H 'sec-ch-ua-platform: "macOS"' \
  --data '{ "token": "phc_URYUdLWjF3NSKKw0futdm41Owob0wAhJLxPku9LaQ0Q", "distinct_id": "019148eb-b78e-7339-bfaf-d6eebfcdeec1", "groups": {} }'
{"errorWhileComputingFlags":false,"featureFlags":{"first-experiment":"control","survey-targeting-a4ea8007c0":true,"survey-targeting-89ad32a148":true,"survey-targeting-20596c69b7-custom":true,"half-of-people":false,"zenhog":true,"surveys-events":true,"survey-targeting-c14f2cb42f":true,"survey-targeting-b35c7ea672-custom":true}}

Doesn't support gzip or any compression yet. Supports both Redis and Postgres for returning flag and team definitions. Only supports a subset of matching logic.

[dmarticus]

not sure if we should call it a key or a token, lol. We'll pick one on which to standardize.

[dmarticus]

real app uses real DB!

[jurajmajerik]

Looking great as far as my Rust knowledge goes :D

Didn't run it, don't have Rust installed yet. Do we have a standard way to do this? Worth updating the docs too: https://posthog.com/handbook/engineering/developing-locally

[dmarticus]

Didn't run it, don't have Rust installed yet. Do we have a standard way to do this? Worth updating the docs too: posthog.com/handbook/engineering/developing-locally

Yeah, the feature flag module readme has some instructions: https://github.com/PostHog/posthog/blob/feat/make-rust-decide-curlable/rust/feature-flags/README.md#L2-L53. But I was going to update the developing locally section once this service is more mature. I'll make a note in the checklist!