[azuremonitorexporter] - Add config validation for duplicate column names

[cijothomas]

AzureMonitor/LogAnalytics fields names must be unique. Given this is coming from config, we can easily validate it upfront.

Few other nits also.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.83%. Comparing base (a2b3698) to head (e0b71ca).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1739      +/-   ##
==========================================
- Coverage   84.09%   81.83%   -2.26%     
==========================================
  Files         470      181     -289     
  Lines      136558    51421   -85137     
==========================================
- Hits       114836    42080   -72756     
+ Misses      21188     8807   -12381     
  Partials      534      534              

Components	Coverage Δ
otap-dataflow	∅ <ø> (∅)
query_abstraction	80.61% <ø> (ø)
query_engine	90.22% <ø> (-0.17%)	⬇️
syslog_cef_receivers	∅ <ø> (∅)
otel-arrow-go	53.50% <ø> (ø)

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[cijothomas]

Closing as discussed with @gouslu who has this incoming in a separate PR.