Add boundary observability dashboard

[zedkipp]

Add a dashboard to the observability repo (https://github.com/coder/observability) that shows HTTP requests audited by agent boundaries within Coder workspaces to provide visibility into workspace network activity.

This dashboard should be a good starting point. It should enable answering questions like:

1. What requests is a particular boundary policy allowing/denying? (note: boundary policy can change across template versions)
2. What requests did boundary audit in a workspace?
3. What requests did boundary audit for a workspace owner?
4. What requests did boundary audit for a specific domain?