Monitoring: Add telemetry collection for Phase 1

[umwelt]

Context

Phase 1 requires telemetry collection for bug reports and performance monitoring during testnet. This fulfills the Phase 1 deliverable: "Gather telemetry, bug reports, and performance data".

Scope

• Collect API request metrics (count, latency, errors)
• Collect identity operation metrics (creation, login, recovery)
• Collect DHT operation metrics (success rate, latency)
• Store metrics in-memory with configurable retention
• Endpoint: GET /api/v1/metrics
• Prometheus-compatible metrics format

Dependencies

None - independent feature

Development Setup

Starting branch: feature/sid-integration (contains latest SID work)

Recommended workflow:

# Clone and checkout base branch
git checkout feature/sid-integration
git pull origin feature/sid-integration

# Create feature branch for this work
git checkout -b feature/telemetry

# Work on implementation
# When done, push and create PR to merge back to feature/sid-integration
git push origin feature/telemetry

Acceptance Criteria

• API request counters by endpoint
• Latency histograms (p50, p95, p99)
• Error rate tracking by endpoint
• Identity operations tracked (creates, logins, recoveries)
• DHT operation success/failure rates
• Metrics endpoint returns Prometheus format
• In-memory storage with 24h retention
• Unit tests for metric collection
• Documentation on available metrics

Technical Notes

• Use prometheus client library
• Metrics: http_requests_total, http_request_duration_seconds, identity_operations_total, dht_operations_total
• Keep metrics lightweight (no PII in labels)
• Support for external monitoring systems (Grafana, etc.)
• API handlers location: zhtp/zhtp/src/api/handlers/