[cmd/mdatagen]Allow custom mapstructure tags for generated config struct fields

[jkoronaAtCisco]

Description

• Adds a mapstructure field to the go_struct schema annotation, allowing schema authors to override the generated mapstructure struct tag for both allOf entries and named properties.
• Without a custom tag, allOf entries still default to mapstructure:",squash" and properties use the property name — no change to existing behavior.
• Enables use cases like the OTLP receiver pattern where an embedded struct should be exposed as a named field (e.g. mapstructure:"protocols") rather than squashed.

Example:

  config:
    type: object
    allOf:
      - $ref: protocols
        go_struct:
          mapstructure: "protocols"

Link to tracking issue

Fixes #15155

Testing

Covered change with corresponding unit tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.31%. Comparing base (22a5e61) to head (d153f56).
⚠️ Report is 37 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15157      +/-   ##
==========================================
- Coverage   91.31%   91.31%   -0.01%     
==========================================
  Files         699      699              
  Lines       44939    44942       +3     
==========================================
+ Hits        41036    41037       +1     
- Misses       2759     2760       +1     
- Partials     1144     1145       +1     

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[dmitryax]

This is not an intuitive user interface. We should not require any mapstructure tag customization for a simple grouping. I'd imagine this to be handled by something like this

  config:
    type: object
    properties:
      protocols:
        $ref: protocols

[jkoronaAtCisco]

This is not an intuitive user interface. We should not require any mapstructure tag customization for a simple grouping. I'd imagine this to be handled by something like this

  config:
    type: object
    properties:
      protocols:
        $ref: protocols

@dmitryax
This schema would produce following go code:

type Config struct {
   Protocols Protocols `mapstructure:"protocols"`
}

but what we need to achieve is:

type Config struct {
  Protocols `mapstructure:"protocols"`
}

(This is real example from OTLP receiver component config)

So it's kind of mix:

• Embedded struct in go
• But named field in collector config

This is currently not possible with config schemas and mdatagen.

[dmitryax]

type Config struct {
  Protocols `mapstructure:"protocols"`
}

this seems wrong to me. I've created #15178 to address that

[jkoronaAtCisco]

This issue will be solved in the other way, as described #15155 (comment)