chore: Add metric type for Prometheus

[fregataa]

Add ai.backend.common.types.PromMetric and ai.backend.common.types.PromMetricGroup.
Doc: Prometheus exposition formats shows the return schema of metric APIs.
This is an example format of metrics.

# HELP metric_name ADDITIONAL-HELP-MESSAGE.
# TYPE metric_name primitive_type
metric_name{label_name=label_value} metric_value # Each PromMetric object creates one line.
metric_name{label_name=label_value} metric_value
...

A PromMetricGroup object creates one metric format and PromMetric object represents each metric value line of metric formats.

Checklist: (if applicable)

• Milestone metadata specifying the target backport version

[fregataa]

• feat: Add Prometheus-compatible manager stat API #2567
• feat: Add manager status report bgtask #2566
• feat: Introduce models.health module that treats manager status #2565
• chore: Add metric type for Prometheus #2833 👈
• main

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