[APM] Use oldest exit span instead of newest

[dgieselaar]

For discovering connections, select the oldest span instead of the newest. This increases the possiblity of the relevant transaction being indexed into Elasticsearch at query time. If this tranasaction isn't there, exit spans might not be appropriately resolved to the downstream service, as reported by @felixbarny and @LucaWintergerst.

[elasticmachine]

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

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 4674f3c
• Storybooks Preview

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[sorenlouv]

Thanks for the quick fix. Do we need to update any tests for this (and if not, is that ok?)

[dgieselaar]

@sqren:

Thanks for the quick fix. Do we need to update any tests for this (and if not, is that ok?)

This issue exhibits itself only when there are indexing delays (between the exit span and entry transaction). So it's quite hard to replicate with an Elasticsearch archive, e.g. you can't use a specific time range. My feeling is that it's not worth the overhead of a (very specific) API test, what do you think?

[sorenlouv]

My feeling is that it's not worth the overhead of a (very specific) API test, what do you think?

Makes sense 👍

[kibanamachine]

Friendly reminder: Looks like this PR hasn’t been backported yet.
To create backports run node scripts/backport --pr 113133 or prevent reminders by adding the backport:skip label.