Add prometheus counter for unique project_identifiers in the Task Get API

The following discussion from !160240 (merged) should be addressed:

• @DylanGriffith started a discussion: (+2 comments)

  Similar to my comment on your other MR I think it would be valuable to add a prometheus counter for every time you see this happen? We can still stop sending duplicates but I think having a counter in prometheus telling us how often this is happening might be useful.

  Feel free to extract this into followup if you're unsure or think it will take too much extra work.