Updating end of interval Flex metrics publishing

[mathewc]

Currently, we only calculate and emit activity metrics during periods of function inactivity. This works well when relatively short lived invocations start stop often during metering windows. However, this doesn't work well in cases where functions are long running (e.g. longer than the default 5ms metering interval), or when invocations are very frequent such that they overlap and there's rarely a period of inactivity. In such cases, we can go many metering intervals without emitting metrics, continuing to accumulate in memory. Eventually the correct duration will be emitted, but it's all shoved into a single interval. In addition to making metrics reporting spiky, this also makes it more likely that metrics may be lost, e.g. if the process dies abruptly.

Instead, we want to ensure that during any interval, we calculate and emit metrics for any outstanding activity in that interval. In this way, even if there are no idle periods, metrics are progressively emitted.

Pull request checklist

IMPORTANT: Currently, changes must be backported to the in-proc branch to be included in Core Tools and non-Flex deployments.

• Backporting to the in-proc branch is not required
  • Otherwise: Link to backporting PR
• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• My changes do not require diagnostic events changes
  • Otherwise: I have added/updated all related diagnostic events and their documentation (Documentation issue linked to PR)
• I have added all required tests (Unit tests, E2E tests)

Additional information

Additional PR information

[paulbatum]

This batch of changes looks good, but I did find one thing I'm not sure about in the existing logic.