Cody completions: Add metrics for completion events

[philipp-spiess]

Closes #51347

This adds metrics for inline and multiline suggestions so we can know how often we trigger requests, how often they are presented to our users, and what the acceptance rate is (note: acceptance rate doesn't work for multiline suggestions yet as we only allow copy-pasting for this and don't have a distinct UI action for accepting)

New events added:

• CodyVSCodeExtension:completion:started (type: 'inline' | 'multiline')
  • Measure when we start a request to the backend for new suggestions (and in turn hit the LLM APIs). I wanted to add this so we can calculate how often these are aborted.
• CodyVSCodeExtension:completion:suggested (type: 'inline' | 'multiline')
  • Measures when we receive a result from the backend and are displaying those to the user (Note: VS Code does not always present inline suggestions to the user, even if this event fires. We'll have to investigate why that's the case and make sure we don't start a completion unless they will be shown).
• CodyVSCodeExtension:completion:accepted (type: 'inline')
  • Measures how often completions are inserted.

Open question: Logging completion on/off (cc @malomarrec)

It's not clear yet how we can measure total number of people that have suggestions enabled (changing the setting for this is not a discrete event since it can happen outside of our control). We could include the state of the feature flag in other Cody related pings but I really think given the occurence of inline completions, these will swiftly become the most-frequently-used Cody feature (when enabled). This leads me to believe that we should calculate DAUs based on the presence of an inline suggestion event.

Test plan

• Add a console.log to the logging code so we can visualize logged events in the console
• https://user-images.githubusercontent.com/458591/235658549-779b1b4f-0603-49fe-9b47-3ca94a7393ab.mov

[vdavid]

(Disclaimer: Haven't looked into the code, just read the desc) Thought: I think we could still detect copy events on the multiline results if we wanted metrics on those as well.

[philipp-spiess]

(Disclaimer: Haven't looked into the code, just read the desc) Thought: I think we could still detect copy events on the multiline results if we wanted metrics on those as well.

Yea I was thinking the same but I’m worried about over-counting here if users copy more than once. Perhaps we can make sure to only trigger it once for a set of suggestions though. I can look into that.

[philipp-spiess]

@vdavid Hm it's not so easy as just adding a copy listener. It is possible to monkey-patch the default copy behavior though: https://stackoverflow.com/a/44812797 but I’m hesitant to do that just now. I'd like to merge this without tracking usage of multiline completion and look into the actual usage first. My hunch is that more people use inline completions over multiline completions and we should optimize for the right thing!