Gitweb understands git tags and displays them when they are associated with a commit so as to be helpful to people browsing the source or logs in gitweb. I am not sure if there is a configuration to disable that in gitweb or Gerrit, but it happens by default. It is indeed metadata, but it is not from the commits, it is the other way around, it is from the tags (the tags are the metadata). Gitweb has to read all the tags to see if any of them happen to point to the commits being displayed. Commits are immutable, and tags are applied after commits are created, so commits "know" nothing of the tags pointing to them,
-Martin