Add flag key on Tracking Hook error stage

[danjuv]

It seems when a flag is successfully evaluated with the tracing hook added, we record the flag's key (and other data) in the span:

    "events": [
        {
            "name": "feature_flag.evaluation",
            "timestamp": "2026-02-07T12:10:02.045159Z",
            "attributes": {
                "feature_flag.key": "test-flag",
                "feature_flag.result.value": "true",
..
            }
        }

But if the flag evaluation errors (e.g. for a FLAG_NOT_FOUND error), the flag key isn't recorded:

    "events": [
        {
            "name": "exception",
            "timestamp": "2026-02-07T12:10:02.044985Z",
            "attributes": {
                "exception.type": "openfeature.exception.FlagNotFoundError",
                "exception.message": "",
                "exception.stacktrace": "openfeature.exception.FlagNotFoundError\n",
                "exception.escaped": "False"
            }
        },
        {
            "name": "feature_flag.evaluation",
            "timestamp": "2026-02-07T12:10:02.045159Z",
            "attributes": {
                "feature_flag.key": "test-flag",
                "feature_flag.result.value": "true",
            }
        }
    

Likely because we only set the flag_key attribute during a successful flag evaluation.

It'd be very helpful to see the associated flag key for an exception in trace time. It seems all we need to do is modify this. The Spec suggests hook context must include flag key

[danjuv]

Perhaps a better approach is to use the Finally stage to attach event attributes, seems to also be more in-line with how other SDKs operate,

[danjuv]

I've taken a stab at fixing this in the linked PR. Happy to align the approach based on what you guys deem best!