As part of #44, we should show when the agent registration occurred and its start/end time in this timeline. This gives users the ability to understand the delta between workspace provisioning completion and the agent startup, which is really an approximate measure of how quickly the compute instance spins up and executes the agent process and how long it takes to call back to the control-plane.

In https://codercom.slack.com/archives/CJURPL8DN/p1729196074736939, Spike and I discussed what would the definition for "agent registration" be. We settled upon: The time between an agent process starting and the successful completion of the call to /api/v2/workspaceagents/me/rpc.
See here: https://github.com/coder/coder/blob/7da231bc9286060e7ad499fa38cb829c4b055a7d/agent/agent.go#L750-L759

The workspace_agents.first_connected_at column seems to satisfy this definition fairly well, and is set here:
https://github.com/coder/coder/blob/5366f2576f690a3f7d3ac1a4efb8dd49cc2e9bd1/coderd/workspaceagentsrpc.go#L330-L338

workspace_agent.created_at is set here which is invoked once a provisioner job has been completed:
https://github.com/coder/coder/blob/fac77f956ec28ad04c0c90750e381e69465d2ae1/coderd/provisionerdserver/provisionerdserver.go#L1782-L1784

Based on this, I think we can simply return the value of created_at (start) to first_connected_at (end) as a reasonable approximation of agent registration time.

Nit: do we want to persist with this "registration time" terminology or is "connection time" more clear?