Instrument metrics via the otel_sdk instead of prometheus_client

[a-thaler]

Description

As in future we want to use OTLP for metric exposure, we should migrate already to the OTEL SDK for metric instrumentation before adding new metrics.

Goal: Operational metrics are exposed as before, but relies on the OTEL SDK allowing to switch to a different exporter in future

Criterias:

• Switch to the OTEL SDK with prometheus exporter, no prometheus dependency is in the code
• Align metadata of the existing metrics by using the same labels as for the central monitoring setup
• Align metric naming to OTEL conventions
• Re-design existing metrics if needed
• Adjust existing dashboards and alerts to the changes