feat: provide an e2e metric tracking workspace build time
We currently support the following metrics:
coderd_provisionerd_workspace_build_timings_seconds- How long the workspace build took end to endcoderd_agentstats_startup_script_seconds- How long the startup scripts took to complete
However, we lack a metric that tracks the operation of starting or creating a workspace from start to finish. This metric would also account for the gap in between a workspace build completing and the agent connecting to the control plane (such as how long it took to download the agent from the control plane)