perf: don't call GetUserByID unnecessarily for Agents metrics loops (#19395)

At the moment, the loop which retrieves and updates the values of the
agents metrics excessively calls `GetUserByID` (a DB query). First it
retrieves a list of all workspaces, filtering out inactive agents (not
entirely clear to me whether this is non-running workspaces, or just
dead agents), and then iterates over those workspaces to get the rest of
the relevant data for the metrics. The next call is `GetUserByID` for
`workspace.OwnerID`. This is unnecessary because the `workspaces_visible` view we pull workspaces from has already been joined with the users table to get the username/name/etc.

This should at least partially resolve
https://github.com/coder/internal/issues/726 
---------

Signed-off-by: Callum Styan <callumstyan@gmail.com>

coderd/workspacestats/reporter.go

@@ -126,13 +126,8 @@ func (r *Reporter) ReportAgentStats(ctx context.Context, now time.Time, workspac
 
 	// update prometheus metrics
 	if r.opts.UpdateAgentMetricsFn != nil {
-		user, err := r.opts.Database.GetUserByID(ctx, workspace.OwnerID)
-		if err != nil {
-			return xerrors.Errorf("get user: %w", err)
-		}
-
 		r.opts.UpdateAgentMetricsFn(ctx, prometheusmetrics.AgentMetricLabels{
-			Username:      user.Username,
+			Username:      workspace.OwnerUsername,
 			WorkspaceName: workspace.Name,
 			AgentName:     workspaceAgent.Name,
 			TemplateName:  templateName,