Add LogRecord observed timestamp field

[jack-berg]

The log data model differentiates between timestamp and observed timestamp.

Timestamp is the time at which the log occurred. Observed timestamp is the time at which the log was recorded in the collection system.

Timestamp is different than observed timestamp if the logs are being processed asynchronously. For example, maybe the logs are being read from a file, or are being processed on a different thread than where they occur. These will be very frequently the same for the SDK, but occasionally they will be different.

We could wait to add setters in the API until users request it, but we really should minimally be including getters on LogRecordData, and we really should populate the LogRecord#observed_time_unix_nano when serializing OTLP payloads.

IMO, we might as well go all the way and add the API setters.

[breedx-splk]

This looks good to me. While I can't imagine that there are huge numbers of use cases for this field in the java side of things, it's best to have the api match the spec for completeness.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 🎉

Comparison is base (8f6f9e7) 91.20% compared to head (b873381) 91.23%.

❗ Current head b873381 differs from pull request most recent head 679c034. Consider uploading reports for the commit 679c034 to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5370      +/-   ##
============================================
+ Coverage     91.20%   91.23%   +0.02%     
- Complexity     4876     4882       +6     
============================================
  Files           549      549              
  Lines         14402    14425      +23     
  Branches       1352     1354       +2     
============================================
+ Hits          13136    13160      +24     
  Misses          877      877              
+ Partials        389      388       -1     

Impacted Files	Coverage Δ
...va/io/opentelemetry/api/logs/LogRecordBuilder.java	60.00% <ø> (ø)
...va/io/opentelemetry/sdk/logs/SdkLogRecordData.java	100.00% <ø> (ø)
.../java/io/opentelemetry/api/logs/DefaultLogger.java	100.00% <100.00%> (ø)
...metry/sdk/testing/assertj/LogRecordDataAssert.java	100.00% <100.00%> (ø)
...ntelemetry/sdk/testing/logs/TestLogRecordData.java	100.00% <100.00%> (ø)
...io/opentelemetry/sdk/logs/SdkLogRecordBuilder.java	98.00% <100.00%> (+0.38%)	⬆️
.../opentelemetry/sdk/logs/SdkReadWriteLogRecord.java	93.75% <100.00%> (+0.20%)	⬆️

... and 1 file with indirect coverage changes

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