Relates tocoder/internal#889.

This PR adds a scaletest runner that simulates a single Coder Connect client receiving workspace updates.

An instance of a workspace updates runner does the following:

• Creates a user, if a session token is not supplied.
• Attempts to repeatedly dial the Coder Connect endpoint, with a configurable (two minutes by default) timeout.
• Once dialed successfully, waits for any other concurrently executing runners to also dial successfully, or timeout (using the barrier).
• Starts a configurable number of workspace builds.
• Waits for that many workspaces to be seen over the workspace updates stream (with a configurable timeout).

Exposes two prometheus metrics:

• workspace_updates_latency_seconds -HistogramVec. Labels ={username, num_owned_workspaces, workspace_name}
  • This is the time between starting a workspace build, and receiving both the corresponding workspace update.
• workspace_updates_errors_total -NewCounterVec. Labels ={username, num_owned_workspaces, action}
  • The number of times a specific action of the runner has failed, per user/client.