fix(syslog source): emit received events/bytes metrics for UDP mode

[sghall]

Summary

• Fixes missing “received” metrics for the syslog source in UDP mode. Closes Syslog source has no received events metrics when the mode is UDP #15687.
• Aligns UDP behavior with TCP and the Component Spec.

Motivation

• In UDP mode, the syslog source did not emit:
  • component_received_events_total
  • component_received_event_bytes_total
  • component_received_bytes_total
• This made observability inconsistent and broke component compliance checks.

Changes

• src/sources/syslog.rs:
  • Emit SocketEventsReceived for UDP with event count and estimated JSON size.
  • Emit SocketBytesReceived for UDP with the datagram byte size.
• Tests:
  • Add test_udp_syslog using the compliance harness to assert:
    • component_received_events_total
    • component_received_event_bytes_total
    • component_received_bytes_total (tagged)
    • component_sent_events_total
    • component_sent_event_bytes_total

Testing

• Focused:
  • cargo test -p vector test_udp_syslog
  • cargo test -p vector 'sources::syslog::test::'

Related

• Issue: Syslog source has no received events metrics when the mode is UDP (Syslog source has no received events metrics when the mode is UDP #15687)
• Thanks for the report and context.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

Notes

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[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[emilychendd]

I have read the CLA Document and I hereby sign the CLA

[datadog-vectordotdev]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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