Platform: Add Prometheus-based instrumentation to snowplow-related codepaths

This issue is directed towards (starting to) add telemetry to the platform implementation to ensure there's sufficient visibility into the different components function.

Our current Snowplow pipelines are monitored via Cloudwatch with the metrics as described here: https://ops.gitlab.net/gitlab-com/gl-infra/config-mgmt/-/blob/main/environments/aws-snowplow-prd/templates/cloudwatch_dashboard.json?ref_type=heads. Not all metrics can be mapped over to the new implementation but should serve as inspiration nonetheless.