[otlp] update proto to v0.18.0

[codeboten]

This change updates the proto to v0.18.0

In this change is also an update to the Sum field of ExponentialHistogramDataPoint to make it optional, which is consistent with HistogramDataPoint.

Fixes #5528
Fixes #5527

[codecov]

Codecov Report

Merging #5530 (ea75920) into main (6abe186) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head ea75920 differs from pull request most recent head c053e1c. Consider uploading reports for the commit c053e1c to get more accurate results

@@            Coverage Diff             @@
##             main    #5530      +/-   ##
==========================================
- Coverage   90.84%   90.81%   -0.04%     
==========================================
  Files         191      191              
  Lines       11362    11365       +3     
==========================================
- Hits        10322    10321       -1     
- Misses        814      817       +3     
- Partials      226      227       +1     

Impacted Files	Coverage Δ
pdata/internal/generated_pmetric.go	97.35% <100.00%> (+0.01%)	⬆️
pdata/internal/common.go	94.61% <0.00%> (-0.77%)	⬇️
service/command.go	80.00% <0.00%> (-0.77%)	⬇️
service/flags.go	88.46% <0.00%> (ø)

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[bogdandrutu]

Please rebase

[codeboten]

@bogdandrutu done