Capture FQDN in APM agents

[SylvainJuge]

Summary

Specification change for #793 .

APM agents should capture the FQDN instead of just the host name, APM Server will handle compatibility with current behavior through configuration (as it's a breaking change).

Fixes #794

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Checklist

• [x] validate the FQDN heuristic on Windows
  • powershell heuristic tested on a Windows VM, custom DNS suffix set through System Properties > Computer Name > Change > More > Primary DNS suffix of this computer, then reboot. When set the System Properties > Computer Name should show the FQDN name in Full computer name field.
• [x] compatibility with older APM server versions: keep sending the current detected_hostname for APM server < 8.9 for the agents that did not already used the FQDN ?
  • NO: host.name is not used very much in APM, thus it's not considered to be a breaking change: comment
• [x] for Windows, should we include the same heuristic described in #795 ?
  • NO: relying on env. variables or currently logged-in user is not reliable

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• May the instrumentation collect sensitive information, such as secrets or PII (ex. in headers)?
  • [x] No:
  • [x] Why? hostname and domain name are not sensitive information
• [x] Create PR as draft
• [x] Approval by at least one other agent
• [x] Mark as Ready for Review (automatically requests reviews from all agents and PM via CODEOWNERS)
  • Remove PM from reviewers if impact on product is negligible
  • Remove agents from reviewers if the change is not relevant for them
• [x] Approved by at least 2 agents + PM (if relevant)
• [ ] Merge after 7 days passed without objections \ To auto-merge the PR, add /schedule YYYY-MM-DD to the PR description.
• [x] Create implementation issues through the meta issue template (this will automate issue creation for individual agents) already created in #793
• [x] If this spec adds a new dynamic config option, add it to central config. n/a

[trentm]

Q2: Should we also be specifying that the FQDN should be lowercased, as the ECS host.name docs recommend? The libbeat impl is lowercasing.

Another argument for lowercasing, at least for Windows is this from MS docs (https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/hostname):

Environment variable %COMPUTERNAME% usually will print the same string as hostname, but in uppercase.

Aside: Yeah Microsoft for (a) the uppercasing and (b) especially for the "usually".

[SylvainJuge]

Q1: Does the "avoid using DNS" warning/condition apply now that we prefer getting the FQDN?

A1: I think capturing the FQDN will very probably involve a DNS query, while that might add some latency and introduce some flakyness I don't think we have better options. In the cases where it's not reliable the fallback using environment variables (the "standard ones" or our own) could still be used.

Q2: Should we also be specifying that the FQDN should be lowercased, as the ECS host.name docs recommend? The libbeat impl is lowercasing.

A2: we should lower-case, spec has been updated to cover this.

Q3: (Unrelated to FQDN) Do other implementations set both of metadata.hostname and metadata.detected_hostname or do you wait for APM Server version detection to know if the APM server is >=7.4?

A3: for Java at least we do check the server version >= before sending metadata.hostname and metadata.detected_hostname.