perf: don't calculate build times for deleted templates (#21072)

The metrics cache to calculate and expose build time metrics for
templates currently calls `GetTemplates`, which returns all templates
even if they are deleted. We can use the `GetTemplatesWithFilter` query
to easily filter out deleted templates from the results, and thus not
call `GetTemplateAverageBuildTime` for those deleted templates. Delete
time for workspaces for non-deleted templates is still calculated.

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

coderd/metricscache/metricscache.go

@@ -87,7 +87,9 @@ func (c *Cache) refreshTemplateBuildTimes(ctx context.Context) error {
 	//nolint:gocritic // This is a system service.
 	ctx = dbauthz.AsSystemRestricted(ctx)
 
-	templates, err := c.database.GetTemplates(ctx)
+	templates, err := c.database.GetTemplatesWithFilter(ctx, database.GetTemplatesWithFilterParams{
+		Deleted: false,
+	})
 	if err != nil {
 		return err
 	}