feat: support custom properties on exception events

[jonathanlab]

💡 Motivation and Context

Adds a Properties field to Exception, plus a SlogAttrsAsProperties helper and WithPropertiesFn option for the slog capture handler.

Closes #132

• Custom properties on exceptions: Exception now has a Properties field for attaching arbitrary key/value metadata to a captured exception. Custom keys are flattened into the event's properties object alongside the built-in ones. Reserved keys ($lib, $lib_version, distinct_id, $exception_list, $exception_fingerprint, $geoip_disable, system context) are preserved — colliding custom keys are ignored.
• slog custom properties: NewSlogCaptureHandler accepts a new WithPropertiesFn option that returns custom properties per slog.Record. A ready-made SlogAttrsAsProperties helper copies every slog attribute onto the event — the common case is a one-liner.

client.Enqueue(posthog.Exception{
    DistinctId: "user-123",
    Timestamp:  time.Now(),
    Properties: posthog.Properties{
        "environment": "production",
        "retry_count": 3,
    },
    ExceptionList: []posthog.ExceptionItem{
        {Type: "Payment failed", Value: "Card declined"},
    },
})

logger := slog.New(posthog.NewSlogCaptureHandler(baseHandler, client,
    posthog.WithPropertiesFn(posthog.SlogAttrsAsProperties),
))
logger.Error("Payment failed", "payment_id", "pay_123", "amount", 99.99)

💚 How did you test it?

Manually and with with tests.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Added the release label to the PR
• Added a version bump label: bump-patch, bump-minor, or bump-major

[github-actions]

posthog-go Compliance Report

Date: 2026-04-17 07:24:34 UTC
Duration: 230ms

⚠️ Some Tests Failed

0/1 tests passed, 1 failed

---

Feature_Flags Tests

⚠️ 0/1 tests passed, 1 failed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	❌	6ms

Failures

request_payload.request_with_person_properties_device_id

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

[marandaneto]

@jonathanlab i think this will mess with the release steps

posthog-go/.github/workflows/release.yml

Lines 157 to 158 in 0650260

	CHANGELOG_ENTRY=$(awk -v defText="see CHANGELOG.md" '/^## /{if (flag) exit; flag=1} flag && /^##$/{exit} flag; END{if (!flag) print defText}' CHANGELOG.md)
	gh release create "v${NEW_VERSION}" --title "v${NEW_VERSION}" --notes "$CHANGELOG_ENTRY"

can you add this comment as part of the commit msg, pr description etc to avoid errors