Bibliometrics analyses with Pubmed metadata
Publication metadata does not have license restrictions. I work for the European partner of Pubmed - Europe PMC, and you can get metadata for every article available in Pubmed via the open API (https://europepmc.org/RestfulWebService). 

In some countries publication abstract may be under copyright. For UK and USA it is possible to reproduce abstracts if your use of this data falls under "fair dealing claim" (see https://www.copyrightservice.co.uk/ukcs/docs/edupack.pdf).