Add standard observability metrics across all services per ADR-0018

[Copilot]

Implements standard Prometheus metrics across all services according to ADR-0014 (Observability Tools) and ADR-0018 (Service Metrics). Previously, services lacked consistent metrics for version tracking, SLA monitoring, and cache performance.

Changes

Metrics Implementation

Added standard metric sets to 4 services (link, metadata, bff, proxy):

Build Information - Exposes version, commit, build time

// Go services
buildInfo.WithLabelValues(version, commit, buildTime).Set(1)
// Namespace: <service>_application_build_info

SLA/SLO/SLI Metrics

• service_availability_ratio - Uptime tracking for SLA compliance
• http_request_duration_seconds - Response time histogram for SLO monitoring
• error_rate_per_minute - Error counter for reliability SLI

Cache Metrics

• cache_hit_total / cache_miss_total - Cache effectiveness tracking
• Hit ratio calculated as hits / (hits + misses)

Service Metrics

• requests_per_second - Load tracking
• response_time_seconds - Performance histogram
• error_rate_percentage - Error rate gauge

Service Updates

• Go services (link, metadata, bff): Added metrics.go with Prometheus client using promauto
• Proxy service (TypeScript): Added StandardMetrics.ts using OpenTelemetry Metrics API
• Updated all service main files to set build info on startup

Documentation

• docs/tutorial/observability-metrics.md - Complete metrics reference with PromQL examples
• docs/tutorial/observability-upgrade-summary.md - Implementation guide and integration steps
• Updated docs/tutorial/observability.md with references

Integration Requirements

Services require build-time variable injection:

go build -ldflags "-X main.version=$VERSION -X main.commit=$COMMIT -X main.buildTime=$(date -u +%Y-%m-%dT%H:%M:%SZ)"

Metrics are exposed but not yet actively recorded. Applications need to call metric functions in:

• Cache implementations (record hits/misses)
• HTTP middleware (record durations)
• Error handlers (increment error counters)

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com/repos/open-telemetry/opentelemetry-go/releases/latest
  • Triggering command: curl -s REDACTED (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

Upgrade observability in the project

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[sonarqubecloud]

Quality Gate failed

Failed conditions
70.9% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud