add a metric search page, not in the dashboard edit page

[godfreyhe]

Sometimes I only need find one metric, and want to search the metric dynamically in the page, which is more convenient than the dashboard. You need 4 steps to query a metric: 1) find the dashboard in the home page, 2) click in, 3) select the title, 4) click 'edit'). If add a search button in the home page, only one step.

[torkelo]

Similar to this, #264