aggregation/txmetrics: exclude user_agent.original from grouping

[axw]

We need the browser name included in grouping keys for RUM. In the initial implementation I just included user_agent.original, hoping we could get away with having few unique values per aggregation interval. However, even if we do that, we'll still have many metrics docs over a longer period, which will slow down UI queries.

In the short term we should remove user_agent.original from the grouping, which will cause the UI to continue to use transaction docs for the browser metrics. In the longer term, we should add user_agent.name, which requires parsing the User-Agent in apm-server.

See also #3814