[dogstatsd] discard timestamp < 1 in the no-aggregation pipeline.

[remeh]

What does this PR do?

Reject timestamp < 1 in the no-aggregation pipeline.

Motivation

Even if not expected, avoid weird edge-cases down the road in other pipelines.

Describe how to test/QA your changes

• Start the Agent
• Send a metric with a valid timestamp: echo -n "custom_metric:60|g|#env:test,dev:remy|T$(( $(date +%s) - 7200 ))" | nc -4u -w0 127.0.0.1 8125
• Send a metric with an invalid timestamp: echo -n "custom_metric:60|g|#env:test,dev:remy|T0" | nc -4u -w0 127.0.0.1 8125
• Send a metric with an invalid timestamp: echo -n "custom_metric:60|g|#env:test,dev:remy|T-1" | nc -4u -w0 127.0.0.1 8125
• Generate a flare
• in expvars/dogstatsd, validate that MetricPackets equals 1 and that MetricParseErrors equals 2

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• 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.
• A release note has been added or the changelog/no-changelog label has been applied.
• Changed code has automated tests for its functionality.
• Adequate QA/testing plan information is provided if the qa/skip-qa label is not applied.
• At least one team/.. label has been applied, indicating the team(s) that should QA this change.
• If applicable, docs team has been notified or an issue has been opened on the documentation repo.
• If applicable, the need-change/operator and need-change/helm labels have been applied.
• If applicable, the k8s/<min-version> label, indicating the lowest Kubernetes version compatible with this feature.
• If applicable, the config template has been updated.

[scottopell]

Good QA plan! Small nit on a comment, but lgtm