Change Policy.id from UUID struct to string

[annacai21]

Policy.id was a fixed 16-byte representation, split into two uint64s as a workaround for FlatBuffers' Go codegen not supporting fixed-size arrays in structs at the time.

That representation only works for real UUIDs. We want id to also carry the sentinel value "hardcoded" for built-in/hardcoded policies (as opposed to Remote Config policies, which get a real per-rule UUID), so a single field can identify the origin of any policy regardless of source.

[datadog-prod-us1-3]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 21.66%

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: a95b06c | Docs | Datadog PR Page | Give us feedback!}

[natitsechanski]

lgtm

[iamluc]

Are you sure of the backward compatibility of this change?

Or maybe it's not filled by the UI? (https://github.com/DataDog/dd-go/blob/prod/remote-config/apps/rc-schema-validation/schemas/apm-policies.json#L193-L196)

[annacai21]

Are you sure of the backward compatibility of this change?

Or maybe it's not filled by the UI? (https://github.com/DataDog/dd-go/blob/prod/remote-config/apps/rc-schema-validation/schemas/apm-policies.json#L193-L196)

@iamluc Good catch, I have a PR that updates id in apm-policies.json from dd_wls_UUID to string here: https://github.com/ddoghq/dd-go/pull/137. That said, this change here is safe on its own today as nothing currently populates a string id in production because the UI currently never fills it in. So every real policy currently has id simply absent, which validates fine either way.