Failing expansion of map string values that look like another type

[dmitryax]

Describe the bug
The issue is similar to #10552 (comment) still can be hit if an expansion is part of a map value.

Steps to reproduce
Run collector with a config like:

receivers:
  otlp:
    protocols:
      grpc:
      http:

exporters:
  otlphttp:
    traces_endpoint: "example.com:4317"
    headers:
      "AUTH-TOKEN": "${AUTH_TOKEN}"

service:
  pipelines:
    traces:
      receivers: [otlp]
      exporters: [otlphttp]

and AUTH_TOKEN env var set to 123

What did you expect to see?
Collector to start.

What did you see instead?
Collector fails with

Error: failed to get config: cannot unmarshal the configuration: decoding failed due to the following error(s):

error decoding 'exporters': error reading configuration for "otlphttp": decoding failed due to the following error(s):

'headers[AUTH-TOKEN]' expected type 'configopaque.String', got unconvertible type 'int', value: '123'
2024/08/20 21:23:23 collector server run finished with error: failed to get config: cannot unmarshal the configuration: decoding failed due to the following error(s):

error decoding 'exporters': error reading configuration for "otlphttp": decoding failed due to the following error(s):

'headers[AUTH-TOKEN]' expected type 'configopaque.String', got unconvertible type 'int', value: '123'

[mx-psi]

#10938 would fix this