Reduce default context expiration time

[vickenty]

What does this PR do?

Reduce default context expiration time from 300s to 20s.

Motivation

Reduce memory usage when tags or metrics received are different across subsequent aggregation intervals.

Additional Notes

Possible Drawbacks / Trade-offs

Describe how to test/QA your changes

Run the agent with telemetry and a dsd socket: DD_DOGSTATSD_SOCKET=/tmp/dsd.sock DD_TELEMETRY_ENABLED=true

Send some dogstatsd metrics once: echo 'foo.bar:1|g' | nc -uU /tmp/dsd.sock

Check that aggregator__dogstatsd_contexts telemetry goes back to zero within 30s.

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.