Expose coderd_workspaces as a Prometheus metric #12462
Description
We currently have workspace_builds, but being able to query the current number of workspaces (including their template and state) can be valuable for understanding usage. We should expose similar fields as the builds metric
# HELP coderd_workspace_builds_total The number of workspaces started, updated, or deleted.
# TYPE coderd_workspace_builds_total counter
coderd_workspace_builds_total{status="failed",template_name="docker",template_version="dazzling_chaum0",workspace_name="workspace-1",workspace_owner="admin",workspace_transition="START"} 1
coderd_workspace_builds_total{status="success",template_name="docker",template_version="dazzling_chaum0",workspace_name="workspace-1",workspace_owner="admin",workspace_transition="START"} 1
coderd_workspace_builds_total{status="success",template_name="docker",template_version="dazzling_chaum0",workspace_name="workspace-1",workspace_owner="admin",workspace_transition="STOP"} 1