Summary of changes

In this PR, we start tracking a traces sampling mechanism (new) in addition to the sampling priority. The sampling mechanism indicates how a sampling decision was made:

• set manually by the user
• using a sampling rate from the agent
• using a sampling rate configured by the user
• sampled by ASM due to security event
• etc

We propagate this information vertically (as tags sent to the Agent) and horizontally (as headers to downstream services).

Reason for change

Tracking sampling decisions helps us provide additional information to users. Horizontal propagation is used to handle individual chunks more efficiently in the backend and to improve the chances that the tag reaches the backend (e.g. if some trace chunks are dropped).

Implementation details

All sampler implementations now return a SamplingDecision instead of a sampling priority (int). In the future we may add more fields, like the sampling rate.

readonly struct SamplingDecision
{
   // sampling priority (-1, 0, 1, 2)
   int Priority;

   // sampling mechanism (0..8), if known
   int? Mechanism;
}

The TraceContext now keeps track of the sampling mechanism in addition to the sampling priority.

When a trace is serialized, we now add the new _dd.p.dm trace tag in addition to the older _sampling_priority_v1 tag. Note that _dd.p.dm uses the new "trace tag" collection. We can migrate _sampling_priority_v1 and other tag to use this feature in a future PR.

Test coverage

• updated ~900 snapshots to add the new _dd.p.dm tag to traces
• added unit test and integration tests for new or updated functionality

Other details

Based on previous work in #2459