Add policy to spans in sampled traces

[kawaiwanyelp]

Component(s)

processor/tailsampling

Is your feature request related to a problem? Please describe.

There is no currently no way to tell by looking at a sampled trace which policy sampled it. If we could, we'd be able to analyze traces by policy, direct traces to different pipelines by policy, and better understand the sampling behavior of this processor.

Describe the solution you'd like

I propose that we add the name of the first top-level policy that returns a positive sampling decision to each span in a trace as an span context attribute. For example, given the following policies:

policies:
  tail_sampling:
    policies:
      - name: probabilistic-policy
        type: probabilistic
        probabilistic:
          sampling_percentage: 10
      - name: http-error-policy
        type: and
        and:
          and_sub_policy:
            - name: error-code-policy
              type: ottl_condition
              ottl_condition: 
                span:
                  - 'IsMatch(attributes["http.response.status_code"], "^[45][0-9][0-9]$")'
            - name: probabilistic-policy
              type: probabilistic
              probabilistic:
                sampling_percentage: 100

And given a trace which includes a span with a 404 status code. Say the probabilistic-policy returned a NotSampled decision, but the http-error-policy returned a Sampled decision. A span in this trace would look like:

{
    "name": "client_request",
    "kind": "SpanKind.CLIENT",
    "attributes": {
        "http.response.status_code": 404,
        "sampling.policy": "http-error-policy"  // new attribute added
    },
    ...
}

Describe alternatives you've considered

• Branch a pipeline into multiple using the forward connector, set up tail-sampling on each branch with different policies, then use the transform processor to add an attribute for the branch a trace was sampled in: Not only does this use more resources as multiple processors have to be set up, it can also result in duplicate samples, and there isn't an easy way to dedup spans/traces in the collector.
• Add the policy to just the root span: This would mean less work for the processor to do, but wouldn't be great if the root span was somehow dropped or lost
• Add all matching policies as a list value: List values are harder to aggregate over than simple string values. List values can also get pretty long for complex policy setups.

Additional context

I'd be willing to create a PR for this if maintainers agree that this is a good idea.

[github-actions]

Pinging code owners:

• processor/tailsampling: @jpkrohling

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[jpkrohling]

I'm not keen on adding another attribute to all spans passing through the tail-sampling processor but I'd be willing to look at PR for this if this is placed behind a feature gate.

[andy-paine-numan]

This would be really useful for debugging new sampling configs in particular.

Add the policy to just the root span: This would mean less work for the processor to do, but wouldn't be great if the root span was somehow dropped or lost

I don't think the risk of losing the root span is very high here since we are using the tailsamplingprocessor, we already know that we've ingested the root span into our collector pipeline for the policies to run. Given this would mean only adding a single resource attribute/attribute rather than adding attributes to every span, this feels like a good solution to me.