Add preview_datadog_agent_sampling

[BrynCooke]

Respect x-datadog-sampling-priority (PR #6017)

This PR consists of two fixes:

Datadog priority sampling resolution is not lost.

Previously a x-datadog-sampling-priority of -1 would be converted to 0 for downstream requests and 2 would be converted to 1.

The sampler option in the telemetry.exporters.tracing.common.sampler is not datadog aware.

To get accurate APM metrics all spans must be sent to the datadog agent with a psr or sampling.priority attribute set appropriately to record the sampling decision.

preview_datadog_agent_sampling option in the router.yaml enables this behavior and should be used when exporting to the datadog agent via OTLP or datadog native.

telemetry:
  exporters:
    tracing:
      common:
        # Only 10 percent of spans will be forwarded from the Datadog agent to Datadog. Experiment to find a value that is good for you!
        sampler: 0.1
        # Send all spans to the Datadog agent. 
        preview_datadog_agent_sampling: true

By using these options, you can decrease your Datadog bill as you will only be sending a percentage of spans from the Datadog agent to datadog.

Important

Users must enable preview_datadog_agent_sampling to get accurate APM metrics.

Important

The Router does not support in-agent ingestion control.
Configuring traces_per_second in the Datadog agent will NOT dynamically adjust the Router's sampling rate to meet the target rate.

Important

Sending all spans to the datadog agent may require that you tweak the batch_processor settings in your exporter config. This applies to both OTLP and the Datadog native exporter.

By @BrynCooke in #6017

---

Checklist

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

• Changes are compatible¹
• Documentation² completed
• Performance impact assessed and acceptable
• Tests added and passing³
  • Unit Tests
  • Integration Tests
  • Manual Tests

Exceptions

Note any exceptions here

Notes

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

[router-perf]

CI performance tests

• connectors-const - Connectors stress test that runs with a constant number of users
• const - Basic stress test that runs with a constant number of users
• demand-control-instrumented - A copy of the step test, but with demand control monitoring and metrics enabled
• demand-control-uninstrumented - A copy of the step test, but with demand control monitoring enabled
• enhanced-signature - Enhanced signature enabled
• events - Stress test for events with a lot of users and deduplication ENABLED
• events_big_cap_high_rate - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity
• events_big_cap_high_rate_callback - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity using callback mode
• events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• extended-reference-mode - Extended reference mode enabled
• large-request - Stress test with a 1 MB request payload
• no-tracing - Basic stress test, no tracing
• reload - Reload test over a long period of time at a constant rate of users
• step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• step-local-metrics - Field stats that are generated from the router rather than FTV1
• step-with-prometheus - A copy of the step test with the Prometheus metrics exporter enabled
• step - Basic stress test that steps up the number of users over time
• xlarge-request - Stress test with 10 MB request payload
• xxlarge-request - Stress test with 100 MB request payload

[svc-apollo-docs]

✅ Docs Preview Ready

No new or changed pages found.

[bnjjj]

Should we also mention potential perf impacts ?

[BrynCooke]

I've added some documentation around setting the sampler:
https://deploy-preview-6112--apollo-router-docs.netlify.app/configuration/telemetry/exporters/tracing/datadog#sampler-configuration

[BrynCooke]

@shorgi Thank you for all the docs improvements!