RUM timestamp is inconsistent with other agent timestamps

I saw APM Server is slapping its own timestamp on client data https://github.com/elastic/apm-server/issues/723. This adds a timestamp for when the transaction **ends** (plus some network lag). However, the timestamp coming from the Java agent is when the transaction **begins**.

Since APM Server knows the duration of the transaction, does it make sense to subtract the duration from "now" so that the timestamps always represent **begin** time?

The issue names APM Server and the linked issue #723; start by reviewing that discussion and the timestamp and duration behavior described here. Done means deciding, with tests or documented evidence, whether server timestamps should represent transaction start consistently with the Java agent.

Written by the indexing model from the issue text.