[dogstatsd] document dogstatsd protocol v1.3 with timestamp support.

[remeh]

What does this PR do?

Document an incoming change in the DogStatsD protocol introducing a new optional field to set timestamp on metrics.

Additional Notes

Change has not been released yet, I'll keep this PR on-hold until the feature lands a release.

Reviewer checklist

• [ ] Review the changed files.
• [ ] Review the URLs listed in the Preview section.
• [ ] Check images for PII
• [ ] Review any mentions of "Contact Datadog support" for internal support documentation.

[github-actions[bot]]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/remeh/dsd-ts/developers/dogstatsd/datagram_shell