Add support for health metrics

[JonasKunz]

Our classic agent reported several health metrics to gain information about it's state. For Otel we should

• check, which health metrics are already exposed by the SDK and add important ones which are missing
• define, how the metrics will be sent (e.g. investigate the op-amp usecase for this maybe)
• align sem-conv to have the relevant metrics across languages

[JonasKunz]

Current semconv WIP for defining health metrics for spans: open-telemetry/semantic-conventions#1631

We'll therefore use standard exporters and not op-amp for sending health metrics. Existing SDK tools should allow defining a separate exporter for health metrics already, if e.g. a different metrics system shall be used for health metrics.