Autocomplete: implement stage counter logger

[valerybugakov]

• Implements a new autocomplete stage logger, which counts the completion requests that reached different stages of the autocomplete generation pipeline.
• It is very similar to https://github.com/sourcegraph/cody/pull/3070, so I assume that "this event is probably a good candidate to exclude from the Amplitude pipeline" based on Kelsey's comment here. cc @akalia25
• Closes https://linear.app/sourcegraph/issue/CODY-2413/%5Bautocomplete-latency%5D-add-feedback-rate-logger
• Slack thread

Test plan

• Added unit tests
• Ensured that dispose is called and we flush when VS Code is closed

[akalia25]

Thanks for the heads-up @valerybugakov, updated our Amplitude pipeline to not ingest this event.

CC: @kelsey-brown