[bilalebi]

Extend resolver outcome metrics across all resolver endpoints

This PR is the continuation of #41

Summary

Extends product-level Prometheus outcome metrics beyond the legacy URL resolver so monitoring consistently reflects resolver behaviour across the service.
The resolver now exposes:

resolver_requests_total{endpoint, outcome, response_mode}

This complements existing HTTP metrics. HTTP status continues to describe the protocol response, while outcome describes whether the resolver achieved an expected product result.

Endpoint coverage

Endpoint family	endpoint label
/legacy	legacy_url
/id/{stable_id}	stable_id
/rapid/...	rapid
/rapid/id/{stable_id}	rapid_stable_id

Outcomes

Outcome	Meaning
resolved	A current Ensembl destination or resolver result was found
archive_fallback	A legacy request was redirected to an Ensembl archive
interstitial	A browser received the expected legacy URL interstitial (404)
not_found	The request was valid but no destination or result exists
invalid_request	The input could not be processed, for example an invalid Rapid accession
internal_error	An unexpected application, configuration, or dependency error occurred

response_mode distinguishes browser/HTML and JSON/API responses.

Why

Raw HTTP 4xx metrics do not accurately represent resolver health:

HTTP 404 on a legacy browser interstitial
≠
Resolver failure

This metric lets Grafana distinguish expected product outcomes—such as archive fallbacks and interstitials—from genuine internal failures.

Dashboard guidance

Use outcome metrics for resolver-health panels and alerts:

sum(
  increase(resolver_requests_total{
    namespace="ensembl-prod",
    service="resolver-api-svc",
    outcome="internal_error"
  }[$__range])
)

Example overall successful-outcome rate:

(
  sum(
    increase(resolver_requests_total{
      namespace="ensembl-prod",
      service="resolver-api-svc",
      outcome=~"resolved|archive_fallback|interstitial"
    }[$__range])
  )
  /
  sum(
    increase(resolver_requests_total{
      namespace="ensembl-prod",
      service="resolver-api-svc"
    }[$__range])
  )
) * 100

Raw HTTP metrics should remain available for traffic and protocol diagnostics, including framework-level responses such as 405 Method Not Allowed, which do not enter resolver route handlers.

Additional change

Simplified the Rapid invalid-accession error path so each request records exactly one invalid_request outcome, without changing its HTTP behaviour.