Add an opt-in collector config + compose override that forward traces,
metrics, and logs to a Grafana Cloud OTLP gateway alongside the local
backends. Enabling it is a runtime choice (layer the override compose
file) — no xrpld rebuild, base stack unchanged. Credentials come from a
gitignored .env file. Documented in the telemetry runbook and TESTING.md.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>