config: increase `dogstatsd_no_aggregation_pipeline_batch_size` default.

[remeh]

256 was fairly small, it'll be better to send more metrics per payload.

Describe how to test/QA your changes

Skipping the QA, it is just a configuration change.

Reviewer's Checklist

• [x] If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• [x] Use the major_change label if your change either has a major impact on the code base, is impacting multiple teams or is changing important well-established internals of the Agent. This label will be use during QA to make sure each team pay extra attention to the changed behavior. For any customer facing change use a releasenote.
• [x] A release note has been added or the changelog/no-changelog label has been applied.
• [x] Changed code has automated tests for its functionality.
• [x] Adequate QA/testing plan information is provided if the qa/skip-qa label is not applied.
• [x] At least one team/.. label has been applied, indicating the team(s) that should QA this change.
• [x] If applicable, docs team has been notified or an issue has been opened on the documentation repo.
• [x] If applicable, the need-change/operator and need-change/helm labels have been applied.
• [x] If applicable, the k8s/<min-version> label, indicating the lowest Kubernetes version compatible with this feature.
• [x] If applicable, the config template has been updated.