[APM] Remove apm_oss.indexPattern from the docs

[gbamparop]

Remove apm_oss.indexPattern from the documentation as it is no longer used.

[elasticmachine]

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

[gbamparop]

Hi @bmorelli25, could you have a look at this issue please?

[bmorelli25]

@gbamparop Just the apm_oss.indexPattern setting? Can you provide more context as to why this should be removed? We currently link to this setting in the APM API docs: Customize the APM index pattern.

[bmorelli25]

I see the setting was removed from the docs in #114139, and other settings were updated in #113364, but I'm still not sure what context is for this removal.

Should we only mention the Kibana update object API as a way to update the default APM index pattern? Or should the entire Customize the APM index pattern topic be removed?

[bmorelli25]

Opened the two PRs ⬆️. Questions still stand but can be addressed in the PRs. Thanks for bringing this to my attention.

[gbamparop]

@bmorelli25 I think the sections related to this could be removed as the config is being constructed now by concatenating the specified indices. Please see #111133 for more context about the decision. @sqren what do you think?

[sorenlouv]

Yes, what @gbamparop said. The index pattern is now automatically inferred from the indices so setting it manually is no longer needed (nor possible).

[bmorelli25]

Thanks. Reopening this issue until Customize the APM index pattern is removed.

[sorenlouv]

@bmorelli25 I see your two PRs are merged. Can this be closed?

[bmorelli25]

Not yet! I still need to do this #114652 (comment)