feat(v2/events/snowplow) add custom context support to v2 Snowplow

Related to:

• Add support for custom contexts to Labkit v2 Sn... (#103 - closed)
• ADR-012: Usage Data Collection, MR
• Iglu artifact_registry_context/1-0-0
• Iglu gitlab_standard/1-1-8
• !450 (closed) — TrackEventWithContexts and GitLabStandardContext (previous version of this change)

This adds support for adding custom contexts to Snowplow payloads.

Similar to !450 (closed), but does some groundwork to support validating contexts and includes validating gitlab_standard per its iglu schema out of the box for Artifact Registry and other projects using Snowplow in LabKit v2.

How to test

cd v2 && go test -race ./events/snowplow/...

Verify on local with GDK's Snowplow Micro

1. Run gdk config set snowplow_micro.enabled true and gdk reconfigure.
2. Make sure snowplow-micro runs (requires Docker).
3. Run this minimal Go program:

package main

import (
	"log"
	"time"

	"gitlab.com/gitlab-org/labkit/v2/events/snowplow"
)

func main() {
	emitter, err := snowplow.NewEmitter("http://localhost:9091/com.snowplowanalytics.snowplow/tp2")
	if err != nil {
		log.Fatal(err)
	}
	tracker, _ := snowplow.NewTracker("verify-labkit-498", emitter)

	if err := tracker.TrackEventWithContexts(
		"labkit_498_verification",
		nil,
		snowplow.GitLabStandardContext{
			Environment:    "development",
			OrganizationID: 1,
			Realm:          snowplow.RealmSelfManaged,
		},
	); err != nil {
		log.Fatal(err)
	}

	tracker.Flush()
	time.Sleep(2 * time.Second) // emitter's send loop is async; wait briefly
	tracker.Stop()
}

go mod init verify
go mod edit -replace gitlab.com/gitlab-org/labkit/v2=/path/to/your/labkit/v2
go mod tidy
go run main.go

4. Verify the event landed in Snowplow Micro:

  curl -s http://localhost:9091/micro/good | jq '.[] | select(.event.app_id == "verify-labkit-498")'

Or just visit http://localhost:9091/micro/good

You should see one event whose contexts array contains an entry with schema = "iglu:com.gitlab/gitlab_standard/jsonschema/1-1-8" and data.environment = "development", data.organization_id = 1, data.realm = "self-managed".