Rework log fields

[jack-berg]

Solves several of the issues in #3804. Replaces #3811.

This PR includes a couple of changes to log fields, based on conversations in the 11/4 SIG:

• setTraceId(..), setSpanId(..), and setFlags(..) are replaced with setContext(Context)
• getTraceId(), getSpanId() and getFlags() are replaced with SpanContext getSpanContext()
• setAttributes(..) now replaces attributes instead of merging them. This is less surprising.
• setBody(Body) is removed. Only setBody(String) is currently supported. On the read side Body getBody() still returns a Body.
• The nullability of fields has been been addressed. Generally, null is avoid if the response has an alternativel
  • SpanContext getSpanContext() returns SpanContext.getInvalid() if unset.
  • Severity getSeverity() returns Severity#UNDEFINED_SEVERITY_NUMBER if unset.
  • String getSeverityText() is nullable and returns null if unset.
  • String getName() is nullable and returns null if unset.
  • Body getBody() returns Body#emptyBody() if unset.
  • Attributes getAttributes() returns Attributes#empty() if unset.

[codecov]

Codecov Report

Merging #3837 (a234bca) into main (fc68a88) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main    #3837      +/-   ##
============================================
+ Coverage     89.33%   89.38%   +0.04%     
- Complexity     4092     4098       +6     
============================================
  Files           488      489       +1     
  Lines         12630    12629       -1     
  Branches       1229     1231       +2     
============================================
+ Hits          11283    11288       +5     
+ Misses          926      923       -3     
+ Partials        421      418       -3     

Impacted Files	Coverage Δ
...va/io/opentelemetry/sdk/logs/data/LogDataImpl.java	100.00% <ø> (ø)
...lemetry/exporter/logging/SystemOutLogExporter.java	94.11% <100.00%> (ø)
...etry/exporter/otlp/internal/logs/LogMarshaler.java	69.62% <100.00%> (+2.95%)	⬆️
.../java/io/opentelemetry/sdk/logs/SdkLogBuilder.java	100.00% <100.00%> (ø)
...main/java/io/opentelemetry/sdk/logs/data/Body.java	100.00% <100.00%> (ø)
...java/io/opentelemetry/sdk/logs/data/EmptyBody.java	100.00% <100.00%> (ø)
...io/opentelemetry/sdk/logs/data/LogDataBuilder.java	100.00% <100.00%> (ø)
...emetry/api/baggage/propagation/PercentEscaper.java	79.69% <0.00%> (-4.52%)	⬇️
...telemetry/sdk/trace/export/BatchSpanProcessor.java	90.07% <0.00%> (-0.71%)	⬇️
... and 7 more

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[anuraaga]

Looks much nicer thanks!