Add agent_type in register agent event in Rails

The following discussion from !2488 (merged) should be addressed:

• @timofurrer started a discussion:

  I assume that this will have impact on the downstream usage event data, right? It'll impact existing things like we plan for https://gitlab.com/gitlab-data/product-analytics/-/issues/2760+. However, good thing is, that I think we don't have any yet 😄

  Anyways, I think we have to add agent_type in Rails here if you are not already on top of this.