Improve live traces observability using counters

What does this MR do?

This merge request adds a couple of labels to gitlab_ci_trace_operations_total Prometheus counter:

1. Increment streamed operation when a new trace arrives
2. Increment appended operation when we write to a live trace chunk
3. Increment chunked operation when a new live trace chunk gets created

It also adds gitlab_ci_trace_bytes_total counter that will allow us to calculate the rate of trace data transferred.

Conformity

• Code review guidelines

Issues

Closes #227182 (closed)