Update telemetry systemCode

[camillecroci]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context.

Summary

Reliability is working on a new client metrics server and Ivo was very proactive in adding the tool kit side of this project but a WIP means that some change might still happen. And friends, they happened. We are now requiring a systemCode at the root level of the event (so same as the namespace) which means the telemetry code needed a bit of refactoring to provide it. From my understanding, the systemCode is not a required property and could be undefined when using that little magical guessSystemCode so I took on me to provide a default one (dotcom-tool-kit) but I am open to suggestion.

Relevant motivation

I was very motivated.
But also it is for tool kit to be up to date with our latest change without having to do anything about it (apart from reviewing this pr I guess)

A bit more context

This was Ivo PR to add the metrics client.

Checklist:

• My branch has been rebased onto the latest commit on main (don't merge main into your branch) (although depending on how long this takes to be merged, I might need to rebase again)
• My commit messages are conventional commits, for example: feat(circleci): add support for nightly workflows, fix: set Heroku app name for staging apps too