Telemetry: number of points dropped

[ogaca-dd]

What does this PR do?

This PR reports the number of points dropped.

Motivation

Additional Notes

Reviewing commit by commit is recommended.

Possible Drawbacks / Trade-offs

Describe how to test/QA your changes

• Disable internet connection
• Start the Agent with the following configuration

use_v2_api.series: true
forwarder_storage_max_size_in_bytes: 10000
forwarder_retry_queue_payloads_max_size: 10000
telemetry.enabled: true

• Wait until http://127.0.0.1:5000/telemetry file_storage__points_dropped_count is greater than 0
• Restore internet connection
• Wait for a minute
• Check the telemetry file_storage__points_dropped_count is equals to the gauge metric datadog.agent.point.dropped

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.