feat(s2n-quic-core): implement nominal timers

[camshaft]

Description of changes:

This change implements nominal timers for the aggregate metrics. It also adds a checkpoint and nominal_checkpoint attribute to events, which allows them to record how long it took for the event to fire since the start of the connection.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[Mark-Simulacrum]

Just to check, we auto-unit durations?

[camshaft]

oh we default to None... lemme fix that. good catch!

[Mark-Simulacrum]

I wonder if time since connection start is the right metric... Time since previous checkpoint might be more useful? Otherwise e.g. slow p100 on one phase will be hard to tell apart from sometimes slow across all the phases (when looking in aggregate)

Essentially time in phase vs. time overall

[camshaft]

I wonder if time since connection start is the right metric...

Hmm... I think it probably depends on what you're trying to do. For example, if you want to know average latency at which dc_state_changed.secrets_ready is hit, you'd have to sum up all of the previous states since then to get that.

But I can definitely see another type of measurement that records "time since last update"