I created a custom TopoJSON map of 2014 US CBSAs as defined by the U.S. Census Bureau for anyone to reuse. Simply copy the URL below to the ‘TopoJSON URL’ field in the editor window as shown in the screenshot below. Make sure you have selected ‘Static Map Regions’ in the visualization pane and not the default map icon.

The map should render in your browser as long as the field in your query contains CBSA or MSA (Metropolitan Statistical Area) codes. This TopoJSON file excludes Hawaii and Alaska.
If you need a custom map that includes HI, AK and US territories, you can use other custom TopoJSON files I created in a public Gist on Github here. You will have to copy the raw Gist URL to rawgit in order to host the TopoJSON file. Then simply copy the production URL into the ‘TopoJSON URL’ field in Looker. Refer to this discourse for a more detailed description of how to host a TopoJSON file on rawgit.