docs: Grafana dashboard for Soroban billing observability

[Ope4196]

PR: Grafana Dashboard for Soroban Billing Observability

Summary

Adds a committed Grafana dashboard JSON (docs/dashboards/soroban-billing.json) so on-call engineers can see Soroban billing deduction latency (P50/P95), error category breakdown, and call rate out of the box — no manual panel creation required.

Changes

New files

• docs/dashboards/soroban-billing.json — Grafana 11.5.2 dashboard with three rows: Deduction Latency, Error Category Breakdown, Call Rate & Throughput
• docs/dashboards/README.md — Documents metric names, provenance, error category → HTTP status mapping, bucket boundaries, SLO thresholds, and import instructions

Modified files

• README.md — Observability section now links to both dashboards under docs/dashboards/

Dashboard panels

Row	Panels
Deduction Latency	P50/P95 line chart, P50 stat, P95 stat, bucket distribution bars
Error Category Breakdown	Rate by HTTP status (proxy for SorobanRpcErrorCategory), total error bar chart
Call Rate & Throughput	Total call rate, success rate gauge

Metric provenance

Metric	Source file
billing_deduct_duration_seconds	src/metrics/registry.ts — recorded by billingDeductHistogramMiddleware
http_requests_total	src/metrics.ts — recorded by metricsMiddleware

Labels used: route="/api/billing/deduct", status_code (maps to SorobanRpcErrorCategory).

Validation

The JSON can be validated with:

# Parse check
node -e "JSON.parse(require('fs').readFileSync('docs/dashboards/soroban-billing.json','utf8')); console.log('valid')"

Security

• No private data baked in (no hardcoded IPs, tokens, or secrets)
• Datasource UID is a $datasource template variable — resolves at import time
• Grafana version pinned to 11.5.2 in __requires

closes #415

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