Add usage gauges for Datadog exporter sub-features (ROUTER-1962)

[SharkBaitDLS]

The native Datadog trace exporter's documented deprecation in favor of OTLP needs usage data to plan against, but today only exporter on/off is measurable (the opt.tracing.datadog attribute). This adds seven attributes to the existing apollo.router.config.telemetry gauge recording which Datadog-specific options are explicitly configured:

Attribute	Config path
opt.tracing.datadog.enable_span_mapping	telemetry.exporters.tracing.datadog.enable_span_mapping: true
opt.tracing.datadog.fixed_span_names	...datadog.fixed_span_names: true
opt.tracing.datadog.resource_mapping	...datadog.resource_mapping (set)
opt.tracing.datadog.span_metrics	...datadog.span_metrics (set)
opt.tracing.datadog.sampler	...datadog.sampler (value)
opt.tracing.common.preview_datadog_agent_sampling	...tracing.common.preview_datadog_agent_sampling: true
opt.tracing.propagation.datadog	...tracing.propagation.datadog: true

Semantics follow the existing pattern: attributes reflect explicit operator configuration (validated_yaml), not materialized defaults. No new instrument or mechanism — this extends the existing populate_config_instrument! block.

Targeting dev so the gauges reach the 2.x line, where most Datadog exporter usage lives; it reaches dev-v3.x through the regular forward-port.

ROUTER-1962

Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• PR description explains the motivation for the change and relevant context for reviewing
• PR description links appropriate GitHub/Jira tickets (creating when necessary)
• Changeset is included for user-facing changes
• Changes are compatible¹
• Documentation² completed
• Performance impact assessed and acceptable
• Metrics and logs are added³ and documented
• Tests added and passing⁴
  • Unit tests
  • Integration tests
  • Manual tests, as necessary

Exceptions

• Documentation: apollo.router.config.* gauges are internal usage instrumentation and are not part of the documented user-facing metrics surface; no doc change needed.
• Integration tests: n/a — behavior is fully covered by the config-metrics snapshot unit test (configuration::metrics::test::test_metrics), consistent with all other attributes on this instrument.
• Performance: additive attributes evaluated via jsonpath once per startup/config reload only; no per-request cost.

Notes

🤖 Generated with Claude Code

Footnotes

1. It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this. ↩

2. Configuration is an important part of many changes. Where applicable please try to document configuration examples. ↩

3. A lot of (if not most) features benefit from built-in observability and debug-level logs. Please read this guidance on metrics best-practices. ↩

4. Tick whichever testing boxes are applicable. If you are adding Manual Tests, please document the manual testing (extensively) in the Exceptions. ↩

[apollo-librarian]

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