I am trying to build a simple workflow using the new Flow app in SharePoint Online. Need to find a way to turn on Flow for selected docs in my library, but not all. Have added a mandatory site column 'Activate Update Doc WFlow'using a term set with a Yes/No. Ideally would like to trigger a flow so that when a doc is tagged Yes, a wflow is triggered after 1 year, where the document author (another site column) is emailed a reminder to update the document content. Is this possible?
This currently not possible with Flow. Only list triggers expose custom metadata properties. It is currently on the development team's backlog, however, and should be available in the future. You can vote for this feature here, which may make it a higher priority (pure speculation, I have no evidence/proof this will do anything now that it is a planned item).
wjervis has the right answer. But to add a bit, you can actually use a "when a new item is created" with a document library, and then you'll have access to all metadata. If you try this, just type in the library display name in the "list name" field. (libraries don't show up on the list, but again, this will work if you manually type it in). But, don't get your hopes up, as while going this route does provide you with the metadata fields, you then won't have access to any of the file level fields, such as the file path, file name, etc.
Edit: sorry, this isn't quite enough info. See my comment below for one more step to get access to the values.