feat(api-logs): add ObservedTimestamp to LogRecord

[llc1123]

Which problem is this PR solving?

Adds ObservedTimestamp to LogRecord according to the specification.

Fixes #3757

Short description of the changes

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Existing tests

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added

[codecov]

Codecov Report

Merging #3787 (9c8c5ce) into main (9fe1f7d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 9c8c5ce differs from pull request most recent head aeb3eb8. Consider uploading reports for the commit aeb3eb8 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3787   +/-   ##
=======================================
  Coverage   92.95%   92.95%           
=======================================
  Files         295      295           
  Lines        9038     9039    +1     
  Branches     1842     1843    +1     
=======================================
+ Hits         8401     8402    +1     
  Misses        637      637           

Impacted Files	Coverage Δ
...erimental/packages/api-logs/src/types/LogRecord.ts	100.00% <ø> (ø)
...mental/packages/otlp-transformer/src/logs/index.ts	100.00% <ø> (ø)
experimental/packages/sdk-logs/src/LogRecord.ts	97.82% <100.00%> (+0.02%)	⬆️

[dyladan]

Generally looks good but you have to be very extra sure you want hrTime if you're going to use it. I personally would recommend using Date.now