[pdata] Deprecate FlagsStruct in favor of Flags

[TylerHelmuth]

Description:
Deprecates FlagsStruct in favor of Flags

Link to tracking Issue:
Related to #5443

Testing:
Unit tests

[TylerHelmuth]

/cc @bogdandrutu

[codecov]

Codecov Report

Merging #5842 (66901a3) into main (8145eb7) will decrease coverage by 0.10%.
The diff coverage is 72.41%.

@@            Coverage Diff             @@
##             main    #5842      +/-   ##
==========================================
- Coverage   91.82%   91.71%   -0.11%     
==========================================
  Files         196      195       -1     
  Lines       11920    11932      +12     
==========================================
- Hits        10945    10944       -1     
- Misses        768      780      +12     
- Partials      207      208       +1     

Impacted Files	Coverage Δ
pdata/internal/metrics.go	87.59% <52.94%> (-5.44%)	⬇️
pdata/internal/generated_pmetric.go	97.33% <100.00%> (ø)
exporter/otlphttpexporter/otlp.go	80.59% <0.00%> (-3.25%)	⬇️
exporter/exporterhelper/queued_retry.go	95.97% <0.00%> (-0.38%)	⬇️
exporter/exporterhelper/queued_retry_inmemory.go

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