VPN-6799: Add tests for timeToMainScreen metric

[oskirby]

Description

There has been some concern that we might have a regression in our timeToMainScreen performance, which is intended to measure the startup time of the application. To make sure that this isn't some bug in the metric accumulation itself, we should add a functional test to ensure that it's generating sensible data.

Unfortunately, this turned into a bit of a mess. There was a hack in the code that used QML/JS to serialize the metric value for testing, but this fails when handling a histogram metric, like TimingDistributionMetric so this required some refactoring to make it go. In particular:

• Abandon the QML/JS implementation so that we can write our own metric serialization code in C++.
• Add a BaseMetric type to try and simplify how metrics are serialized, and give them a unified interface.
• Add an extra test to make sure that the CustomDistributionMetric we use for RX/TX transfer stats still work.

And after all that is done, we can now check the timeToMainScreen metric as follows:

let timing = await vpn.gleanTestGetValue("performance", "timeToMainScreen", "");

Reference

JIRA issue: VPN-6799
JIRA issue: VPN-6186

Checklist

• My code follows the style guidelines for this project
• I have not added any packages that contain high risk or unknown licenses (GPL, LGPL, MPL, etc. consult with DevOps if in question)
• I have performed a self review of my own code
• I have commented my code PARTICULARLY in hard to understand areas
• I have added thorough tests where needed

[mcleinman]

Looks great, thanks!