[Observability Onboarding] Auto-detect flow: Generated config files are not valid

[flash1293]

Kibana version: serverless/main

Describe the bug: When going through the auto-detect flow, some generated config files like for the system or docker integration are not valid.

Steps to reproduce:

1. Go through onboarding flow
2. Install system integration
3. Errors are shown

Expected behavior:

Should ship data

Errors in browser console (if relevant):

Waiting for healthy status...
curl: (22) The requested URL returned error: 400
Elastic Agent is not healthy.\nCurrent status: FAILED / Invalidcomponentmodel:renderinginputsfailed:condition\\u003cCONDITION\u003e\evaluationfailed:2errorsoccurred:\n\t*conditionline1column0:extraneousinput'\u003c'expecting{TRUE,FALSE,FLOAT,NUMBER,NOT,NAME,STEXT,DTEXT,'(','[','{','$${','${'}\n\t*conditionline1column10:mismatchedinput'\u003e'expecting'('\n\n.\nFor help, please see our troubleshooting guide at https://www.elastic.co/guide/en/fleet/8.13/fleet-troubleshooting.html.

Any additional context:

This is caused by placeholders being added to the generated yaml:

kibana/x-pack/plugins/fleet/server/services/epm/packages/get_template_inputs.ts

Line 276 in a63b939

const placeholder = getPlaceholder(varDef);

e.g.

  - id: system-system/metrics
    type: system/metrics
    streams:
      # System core metrics: Collect System core metrics
      - id: system/metrics-system.core
        data_stream:
          dataset: system.core
          type: metrics
        metricsets:
          - core
        core.metrics:
          - percentages
        system.hostfs: <SYSTEM.HOSTFS> # Proc Filesystem Directory: The proc filesystem base directory.
        tags:
          - <TAGS> # Tags

[elasticmachine]

Pinging @elastic/obs-ux-logs-team (Team:obs-ux-logs)

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[flash1293]

Closed by #199168