SSE connections leaked

[spikecurtis]

We leak SSE connections to /api/v2/workspaceagents/{id}/watch-metadata in at least the following scenarios, since v1.4.2.

• A workspace is restarted while the IDE plugin is tracking it
• You sign into a different Coder deployment with some workspaces running

I have a strong suspicion that #440 is involved, although possibly not the root cause.

Likely the cause of https://github.com/coder/customers/issues/741

How to reproduce

1. Connect to a Coder server as a user running at least one workspace.
2. Monitor the coderd_api_concurrent_requests gauge via the Prometheus endpoint on Coder server.

Either:

1. Repeatedly stop and start the workspace
2. Repeatedly sign into a different Coder deployment and then back into your test deployment

Then:

1. Observer the concurrent requests guage climb with each cycle
2. Exit VSCode, and see it drop.