Metrics: add exporters in Development phase

[inge4pres]

Describe the current state

Stable exporters are to be handled via #15.
The (currently) 2 exporters in Development are

• Prometheus Exporter
• OTel protocol File Exporter

Describe the desired state

Both exporters are included as part of the SDK.
They can either be part of the "built-in" exporters or they could live in their own module until they graduate.

Additional context

For the Prometheus exporter, https://github.com/karlseguin/metrics.zig exists to expose metrics the Prometheus format, but the mapping between OTel model and library model might not as easy to deliver.
Consider using only parts of the library to generate the textual representation; more importantly, the exporter MUST have an HTTP server to expose the metrics (it will be scraped).