Documents two recently merged spiceai/spiceai PRs and adds a focused DataDog OTLP ingestion guide.

Related runtime PRs

• feat(otel): apply runtime.telemetry.properties as resource attributes on exported metrics spiceai#10416 — apply runtime.telemetry.properties as resource attributes on exported metrics
• feat(otel): add metric name prefix at runtime.telemetry.metric_prefix spiceai#10418 — add runtime.telemetry.metric_prefix for namespacing exported metrics

Changes

• website/docs/reference/spicepod/runtime.md — added runtime.telemetry.metric_prefix and runtime.telemetry.properties reference entries, including notes that they apply to all metric readers (Prometheus scrape, cluster OTLP reader, OTLP push exporter) and that OTEL_SERVICE_NAME / OTEL_RESOURCE_ATTRIBUTES act as defaults.
• website/docs/features/observability/index.md — new "Metric Naming and Custom Tags" subsection that links to the new reference entries and to the DataDog guide.
• website/docs/monitoring/datadog/index.md — new "OpenTelemetry OTLP Export" section covering:
  • Minimal OTLP/HTTP push to https://otlp.<site>.datadoghq.com/v1/metrics with DD-API-KEY from a secret store.
  • Namespacing Spice metrics with metric_prefix.
  • Adding custom tags via properties plus the required dd-otel-metric-config: '{"resource_attributes_as_tags": true}' header so resource attributes surface as tags in DataDog.
  • Note that DataDog tags can lag behind metrics by a few minutes.
  • Caution on tag cardinality with a pointer to Metrics without Limits™ (allowlist/blocklist per metric, configured via Metrics Summary or the Metrics API, with the in-app indexed-volume estimator).
  • Full end-to-end example combining all of the above.