x/tools/gopls: add telemetry counters that will help with the v0.17.0 release

[findleyr]

In the v0.17.0 release, we are planning a couple decisions that could benefit from more information:

• 67821

• 65917

We should add telemetry to gopls@v0.16.1 that can help us plan for these changes:

• Keep track of when people accept the telemetry prompt. This will provide signal for what fraction of users opting in to telemetry are doing so as a result of the IDE prompt. (of course we can't keep track of when people don't accept the prompt!)
• Record whether users are setting GOTOOLCHAIN=auto, GOTOOLCHAIN=local, and GOTOOLCHAIN=. That will provide signal for what fraction of users may not have toolchain upgrades available.

EDIT: since this is mentioned in the gopls@v0.16.1 released notes, I wanted to note the following:

• This issue is for local instrumentation only.
• Collection of this data will be filed as a separate public proposal.
• This only affects users who have opted in to sharing telemetry data.

[gopherbot]

Change https://go.dev/cl/595560 mentions this issue: gopls/internal/server: add counters to inform v0.17.0

[gopherbot]

Change https://go.dev/cl/595836 mentions this issue: [gopls-release-branch.0.16] gopls/internal/server: add counters to inform v0.17.0