[APM] Replace @timestamp with timestamp.us

[sorenlouv]

Today APM documents have two timestamp fields:

• @timestamp (millisecond precision)
• timestamp.us (microsecond precision)

Since 7.0 Elasticsearch has supported nanosecond precision for timestamps. We can take advantage of this by upgrading the precision of @timestamp and at the same time drop timestamp.us to save storage.

If the UI is changed to only use @timestamp it will cause a lack of precision when viewing old data (before this change). Therefore it can be seen as a "light" breaking change.

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[axw]

@sqren I did some investigation into this, and I don't think there's much we can do at the moment: elastic/apm-server#1678 (comment)

[sorenlouv]

@axw Thanks for doing that. I'll pull this out of 7.16 then and mark it as blocked. This also means we won't do this before 8.0

[botelastic]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.