feat: add metadata to pyrra_objective

[akselleirv]

• Adds the .spec.description to pyrra_objective
• Adds a prom label to pyrra_objective named info which adds helpful metadata information about the SLO. Inspired from how the Pyrra UI adds latency information which makes it easier to understand what the latency SLO is measuring.

Example of using the new metadata labels in the Grafana dashboard:

Relates to #745

[svyatoslavmo]

@metalmatze Hello! Any chance to get it merged?

[ArthurSens]

You can already add labels to pyrra_objective by adding pyrra.dev/your-label: label value to the SLO definition... isn't the use case already covered here? 🤔