Optimize and track time to first completion

[Tyriar]

CPU profile on exthost:

To get that I closed all terminals, reloaded the window, waited for things to settle, started recording, opened the terminal and typed "cd"` and pressed ctrl+space.

It seems to take about 5 seconds to get completions here. Some things to look into:

• Let's add a telemetry event that reports time to first completion the first time suggest is requested for the window (so it consistently includes extension activation). This is a strong metric to track by OS for the quality of the feature based on how annoying it is waiting for the first one on Windows to show up
• Why is getPwshGlobals called multiple times? Is it a single call but async across multiple tasks?
• Can we parallelize any of this to make it finish faster?

[Tyriar]

Actually maybe we should just track time to completion, at least while it's off by default. With some boolean to indicate whether it's the first one. That should give us more insights and also let us track the first one.