datadog_agent source: support Datadog v3 series metrics intake API

Use Cases

Datadog Agent 7.81.0 changed the default series metrics submission to the new v3 intake API (/api/intake/metrics/v3/series). Agents configured with additional_endpoints/dd_url pointing at a non-Datadog endpoint (such as Vector's datadog_agent source) now get 404s on that endpoint unless v3 is explicitly disabled, since the datadog_agent source only understands the v1/v2 series payload formats.

Without v3 support, any user running a current Datadog Agent that forwards metrics to Vector needs to manually opt back into v2 on their Agent config, which is a surprising extra step and diverges from the Agent's now-default behavior.

Attempted Solutions

As a workaround, the Agent can be configured to keep sending v2 payloads to a specific endpoint:

use_v3_api:
  series:
    endpoints:
      "http://fakeintake-agent:80": "false"
      "http://vector:8181": "false"

tests/e2e/datadog-metrics (see #25789) currently pins its agent_version test matrix away from latest to avoid this rather than fixing the underlying gap.

Proposal

Add support for decoding the v3 series metrics intake payload format in src/sources/datadog_agent, so Vector can receive metrics from Agent installations running with default (v3) settings, without requiring the workaround above. Once implemented, the tests/e2e/datadog-metrics agent_version matrix should have a latest (or otherwise unpinned) leg re-added that exercises the v3 path.

References

• #25789

Start in src/sources/datadog_agent and inspect the existing v1/v2 series payload decoding. Review tests/e2e/datadog-metrics and its agent_version matrix, including the setup described in #25789. Done means Vector accepts Datadog Agent v3 series metrics and the e2e matrix includes a latest or otherwise unpinned leg exercising that path.