feat(backend): implement granular metrics tracking for Multi-currency Exchange Rate Service

[barry01-hash]

Closes #1118

Summary

Adds granular Prometheus metrics tracking for the Multi-currency Exchange Rate Service (path payment quote endpoint), providing observability into quote request volume, latency, asset pair distribution, Horizon API call health, and slippage application.

Closes #[issue_id]

New Metrics

Exchange Rate Quote Requests (exchange_rate_quote_requests_total)

Counter with labels: source_asset, dest_asset, result (success, not_found, error, rate_limited, same_asset, not_pending)

• Recorded at the route handler level for all request outcomes
• Also recorded at the findStrictReceivePaths function level for Horizon call results

Exchange Rate Quote Duration (exchange_rate_quote_duration_seconds)

Histogram with labels: source_asset, dest_asset, result
Buckets: 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10 seconds

• Captures end-to-end latency for quote resolution

Exchange Rate Horizon Calls (exchange_rate_horizon_calls_total)

Counter with labels: operation (strict_receive_paths, load_account), status (success, error)

• Tracks Horizon API call health for the exchange rate service

Exchange Rate Source Account Validation (exchange_rate_source_account_validation_total)

Counter with labels: result (valid, not_found, skipped, error)

• Tracks source account validation outcomes before quote resolution

Exchange Rate Slippage Applied (exchange_rate_slippage_applied_total)

Counter with labels: slippage_pct

• Tracks how often the 1% slippage buffer is applied to quotes

Files Changed

Modified:

• backend/src/lib/metrics.js — Added 5 new metric definitions and registration
• backend/src/lib/stellar.js — Added metrics instrumentation to findStrictReceivePaths()
• backend/src/routes/payments.js — Added metrics tracking to the GET /api/path-payment-quote/:id route handler

Added:

• backend/src/lib/metrics.test.js — 6 tests validating metric definition, registration, and Prometheus output format
• backend/src/lib/stellar-metrics.test.js — 4 tests validating metrics recording for success, not_found, error, and skipped validation paths

Test Results

• 10 new tests all passing
• All existing 855 passing tests unchanged
• 32 pre-existing test failures unchanged (unrelated to this change)

Verification

# Verify metrics are exposed
curl http://localhost:3000/metrics | grep exchange_rate

# Run metrics tests
cd backend && npx vitest run src/lib/metrics.test.js src/lib/stellar-metrics.test.js

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