ddtrace/tracer: support default origin on dynamic config to support Active Tracing telemetry spec

[darccio]

What does this PR do?

Defaults dynamic config origin to default when created.

It also sets one specific case to env_var according to the telemetry API spec (more detail in AIT-8580).

Motivation

Fix failing telemetry system-tests in https://github.com/DataDog/system-tests/pull/2063.

Reviewer's Checklist

• [x] Changed code has unit tests for its functionality at or near 100% coverage.
• [x] System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• [ ] There is a benchmark for any new code, or changes to existing code.
• [ ] If this interacts with the agent in a new way, a system test has been added.
• [x] Add an appropriate team label so this PR gets put in the right place for the release notes.
• [ ] Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

For Datadog employees:

• [ ] If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• [ ] This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[darccio]

@avivenzio-dd Before merging this one, what do you think it's the best course to ensure it's implementing right the spec?

[avivenzio-dd]

@avivenzio-dd Before merging this one, what do you think it's the best course to ensure it's implementing right the spec?

@darccio good call

Can we start up a small service and send some traces using the changes in this branch to a staging org? or is there a way to run the system tests based on this branch? I'm assuming sending some traces is probably easier.

We can verify the telemetry events are correct using their debug logs.

[pr-commenter[bot]]

Benchmarks

Benchmark execution time: 2024-05-03 11:00:34

Comparing candidate commit a08e194cf6aaab3b410b35f95267b95075bc994a in PR branch dario.castane/AIT-9990-default-origin with baseline commit 3a426cad6b642bc0dab36d49c02f8e09b40529e5 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 44 metrics, 0 unstable metrics.

[darccio]

@avivenzio-dd The best way I found is to temporarily modify the GitHub CI config to run the parametric tests from https://github.com/DataDog/system-tests/pull/2063. This week is heavy on ER thus reducing our capacity to do anything more sophisticated.

A first run shows that it's correctly running from the linked PR in system tests, combined with this PR. It failed anyway because we are returning default instead of env_var in a test, so I'm going to fix that I fixed it.

Edit: a second run seems to fail too but no longer due to the origin field's value. Please, can you check it? I think it's because you are not expecting the runtime ID as tag.

[github-actions[bot]]

This PR is stale because it has been open 20 days with no activity. Remove stale label or comment or this will be closed in 10 days.

[avivenzio-dd]

@darccio is there something pending from my side on this? I think the tests should be all updated now.

[darccio]

@avivenzio-dd No, it's on my roof. Let me try to give it a try tomorrow.

[darccio]

@avivenzio-dd I did another try but it fails. I'm having availability issues as I've been reassigned to a R&D project for one or two months. How do you want to proceed?