Modify adjusted MTTP panel to fallback to regular MTTP

Since we created the adjusted lead time metric on 7th August, if we try to calculate the adjusted MTTP for August, it is calculated from 7th August onwards. 1st to 7th is ignored because the adjusted lead time metric does not exist.

This MR modifies the adjusted MTTP panel to use the adjusted lead time metric and fall back to the regular lead time metric if the adjusted metric is not available. This will allow us to see the adjusted MTTP for August.

https://dashboards.gitlab.net/dashboard/snapshot/Xuj7lob0DJTxgR52JqSuHggz3Z7Xedo6