Add missing mapstructure tag for TLS in promcfg/config.go

[pipiland2612]

Which problem is this PR solving?

• Resolves [Bug]: TLS options for Prometheus are leading to crash when specified in a YAML configuration file #7364

Description of the changes

• Add "mapstructure:tls" to promcfg
• Add corresponding unit test

How was this change tested?

• CI

Checklist

• I have read https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
• I have signed all commits
• I have added unit tests for the new functionality
• I have run lint and test steps successfully
  • for jaeger: make lint test
  • for jaeger-ui: npm run lint and npm run test

[pipiland2612]

When I use this configuration:

metric_backends:
  prom:
    prometheus:
      endpoint: https://example.com
      normalize_calls: true
      normalize_duration: true
      token_file_path: /tokens/prometheus_token
      token_override_from_context: true
      tls:
        insecure: false
        ca_file: /tls/ca.crt 

Here's are the logs that I get (I don't have any /ca/ca.crt file):

...
jaeger-1      | 2025-07-23T17:59:26.262Z        info    extensions/extensions.go:69     Stopping extensions...  {"resource": {"service.instance.id": "791e4767-c8f7-4969-b32d-2f4651f56aba", "service.name": "jaeger", "service.version": "v2.8.0"}}
jaeger-1      | 2025-07-23T17:59:26.262Z        info    [email protected]/service.go:336 Shutdown complete.      {"resource": {"service.instance.id": "791e4767-c8f7-4969-b32d-2f4651f56aba", "service.name": "jaeger", "service.version": "v2.8.0"}}
jaeger-1      | Error: failed to start extensions: cannot create metrics reader failed to load TLS config: failed to load CA CertPool File: failed to load cert /ca/ca.crt: open /ca/ca.crt: no such file or directory
jaeger-1      | 2025/07/23 17:59:26 failed to start extensions: cannot create metrics reader failed to load TLS config: failed to load CA CertPool File: failed to load cert /ca/ca.crt: open /ca/ca.crt: no such file or directory

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.45%. Comparing base (4d6ac93) to head (73ff053).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7367      +/-   ##
==========================================
+ Coverage   96.44%   96.45%   +0.01%     
==========================================
  Files         375      375              
  Lines       22871    22871              
==========================================
+ Hits        22058    22061       +3     
+ Misses        615      613       -2     
+ Partials      198      197       -1     

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unittests	95.43% <ø> (+0.01%)	⬆️

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[yurishkuro]

Thanks!