perf(breadcrumbs): use unix epoch for timestamps

[kattrali]

Goal

Improve performance on the breadcrumb storage "hot path" by removing Date formatting. Adds support for millisecond time resolution as a bonus.

Design

Created an encoding for timestamps where the date value is sent using the pattern t[0-9]+, where the number is the count of milliseconds since the epoch and the t indicates that timestamp parsing should be used. The breadcrumb added state event serializes the timestamp into this format, which is what is now persisted to disk in the event of a crash.

Changeset

• Added a new utility function to decode milliseconds from a string into ISO8601 timestamps into a pre-allocated buffer. Kept this in the same file as breadcrumb serialization to avoid exporting an extra symbol or defining the function body in a header, which is not a pattern we currently have. In the event that decoding fails, the original string is then sent as the breadcrumb timestamp.

Questions

What would be the best way to express optimization changes as a part of a PR? We haven't done a lot of perf-specific improvements and we probably need some standardization here. For example, I can diff the bugsnag-benchmarks entry for simple and complex breadcrumbs and include that in the description?

A flame graph of the breadcrumb path is basically the same as before, just chopping all of the date parsing logic from the top. 🪓

Testing

• Altered a unit test to check both the old and new storage formats are decoded correctly
• All e2e tests go through this path now and should get valid timestamps

[lemnik]

LGTM