docs: Add Lore Server metrics and traces reference page

[ryanlitalien]

TL;DR

Adds a source-verified, vendor-neutral OpenTelemetry metrics and traces reference page for Lore Server, resolving EpicGames#48. Docs-only, 2 files, DCO-signed. Draft against the fork main for internal review before any upstream submission.

What

Adds a Reference page, docs/reference/lore-server-metrics.md, that catalogs the OpenTelemetry metrics, spans, and attributes that Lore Server exposes. Also links it from the reference index and cross-links the telemetry settings in the server configuration reference.

Why

The docs state that Lore Server supports metrics, logs, and traces through OpenTelemetry, but there was no reference for which ones. Discovering them meant reading source or wiring up a collector and inspecting the output.

Contents

• How metric names are built (<namespace>.<instrument>, the urc.* / lore.* split, helper-constructor types and units).
• Shared context / success attributes and resource attributes.
• HTTP and gRPC request metrics (OpenTelemetry semantic-convention names).
• The recurring operation_duration pattern, with a namespace-to-context table.
• Storage, QUIC transport, service counters, runtime metrics, and the traces / sampling model.

Notes

• Vendor-neutral, per the docs language canon (no third-party backend names in the main docs).
• Every instrument name, type, unit, and attribute was catalogued and spot-verified against the source. A stability note flags that the legacy urc.* internal instruments can change between releases.
• Follows the Reference doc type and house style: single H1, indicative mood, tables for the surface, valid cross-page anchors.

Review focus

• Accuracy of instrument names and context values.
• Whether the low-level urc.* internal metrics (Tokio runtime, QUIC internals) should be documented in this depth or trimmed.