Improve observability around Live Traces

Description

Live Traces is an important feature that will allows to decouple GitLab CI/CD from dependency on a local shared storage.

It has been built some time ago but the feature is not considered production ready. In order to make it production ready we would need to increase observability around it.

Proposal

1. Implement mechanisms to validate correctness of traces
2. Implement metrics / logs that will make it visible when traces are not valid
3. Implement metrics / logs that will make it easier to understand how the feature performs

/cc @darbyfrey