Report heap snapshot size delta

[mhofman]

What is the Problem Being Solved?

One of the metric we're interested in is the growth of xsnap worker heap snapshots. While we currently report the uncompressed size of snapshots in heap-snapshot-save slog events, it's sometimes difficult to observe the growth rate: a previous snapshot save event is not always available in the same execution. At least one heap-snapshot-load event should be present and contain the uncompressedSize, but that means we'd now have 2 different kind of events to observe

Description of the Design

Remember the uncompressedSize in RAM when loading a snapshot, and compute the uncompressedSizeDelta when saving the snapshot, and report with the other metrics. When making a snapshot after start/upgrade, the delta would be null.

Security Considerations

None

Scaling Considerations

None

Test Plan

Verify new field presence in slog events

Upgrade Considerations

None