feat: add public metrics endpoint

[tim-hm]

Problem

To improve observability and understand the overall health and activity of the network, ⁠nildb nodes need to expose key metrics publicly.

Proposed Solution

• Create a public, unauthenticated endpoint at ⁠/metrics.
• Expose metrics in a standard format compatible with monitoring tools like Prometheus.
• Initial metrics should include:
  • Usage Metrics: ⁠nildb_total_stored_bytes, ⁠nildb_total_secrets_stored, ⁠nildb_total_records_stored
  • Activity Metrics: ⁠nildb_http_requests_total (aggregated all codes and methods), ⁠nildb_active_builders_total

Acceptance Criteria

• A GET request to ⁠/metrics on a node returns a plain text response with the specified metrics.
• The metrics are correctly formatted for Prometheus scraping.

• The endpoint is documented via openapi.

[psofiterol]

This looks great. The one feature that would be good to add is the ability filter the above by builder.

[tim-hm]

I was thinking the endpoint would be public — do we want by builder to be publicly veiwable?

[psofiterol]

I was thinking the endpoint would be public — do we want by builder to be publicly veiwable?

no, that's not a good idea, it shouldn't be. OK, we'll have time to think about this 🙏 .