Telemetry: provide skill name details metric or metric dimension

### What would you like to be added?

An additional label dimension for the `gemini_cli.tool.call.count` and `gemini_cli.tool.call.latency_bucket` metrics which contains the skill name.
Alternatively, separate metrics would also make sense (e.g. `gemini_cli.skill.call.count` and `gemini_cli.skill.call.latency_bucket` )

### Why is this needed?

Measuring skill activation and duration is useful in larger companies where certain skills are shared and one wants to understand their adoption and need for optimisation.

### Additional context

_No response_