Export metrics

[rhamzeh]

Checklist

• I agree to the terms within the OpenFGA Code of Conduct.

Describe the problem you'd like to have solved

As a consumer of the SDK, I would like to hook it to my dashboards to get data on several metrics.

Describe the ideal solution

Integrate the OpenTelemetry Go API library https://github.com/open-telemetry/opentelemetry-go

Expose only from the inner OpenFgaApi. We need to be able to get these metrics:

• The latency of the request, split into
  • time it took since dev called the method until they get a response
  • time it took since the sdk issued a request to the API until it got a response
  • time reported by the server (query_duration_ms header)
• Response codes
• Error codes and method
• Method calls

See here for examples on usage:

• datastore query count in OpenFGA https://github.com/openfga/openfga/blob/4b47d50594b9c7e436d64cf783c0aedba30b940f/pkg/server/list_users.go#L95-L98
• other examples: https://github.com/open-telemetry/opentelemetry-go/tree/main/example/otel-collector