x/tools/gopls: add a debug endpoint to download diagnostic information

[findleyr]

A few patterns have emerged related to troubleshooting gopls issues:

• Someone files a gopls issue, and our first response is to ask them to attach logs. More than half the time (I'd guess), we don't get a follow up response from the person filing the issue.
• Someone encounters a bug, but can't repro in a fresh session and it's too late to capture logs in their current session.
• Someone has memory problems, and they have to grab a heap dump / profile. Again, this is a burden on users that often leads to disengagement.

EDIT: after a couple years, we have new ideas and new problems. Coopting this feature request for the v0.12.0 milestone.

Especially with the new gopls performance work, we need a way to collect debug information from an already running gopls. We've seen that custom commands are a good way to achieve this, as they leverage the LSP protocol and therefore don't need any additional endpoints. A custom debuging command should enable the following:

• Collecting memory statistics (I actually added such a command in https://go.dev/cl/468935)
• Collecting pprof information.
• Collecting exported metrics from the debug package.
• Collecting recent logs from a ring buffer.
  • We should consider including 'trace' level logs that aren't reflected to the LSP client.
• Collecting bugs observed on the server (via bug package)

With #45518, we'll be able to dynamically start the gopls debug server. This means that users should always be able to access a user interface into internals of the running gopls process. We should leverage this to make it easier to collect diagnostic information.

This is the idea we've discussed:
1. add a new endpoint to the debug server that provides a form allowing the user to select different types of diagnostic information (off the top of my head: RPC logs, heap dump, profile, execution traces), and specify a collection period.
2. when this form is submitted, start collecting this information for the specified period.
3. after collection completes, redirect the user to download a zipped payload that they can send to us.