Redefine ExportSpans of SpanExporter with ReadOnlySpan

[MrAlias]

This updates the SpanExporter interface to accept ReadOnlySpans instead of SpanSnapshots. This includes the following changes:

• The SpanStub type and its associated functions were added to the go.opentelemetry.io/otel/sdk/trace/tracetest package. This type can be used as a testing replacement for the SpanSnapshot that was removed from the go.opentelemetry.io/otel/sdk/trace package.
• The ExportSpans method of theSpanExporter interface type was updated to accept ReadOnlySpans instead of the removed SpanSnapshot. This brings the export interface into compliance with the specification in that it now accepts an explicitly immutable type instead of just an implied one.
• Removed the Tracer and IsRecording method from the ReadOnlySpan in the go.opentelemetry.io/otel/sdk/trace. The Tracer method is not a required to be included in this interface and given the mutable nature of the tracer that is associated with a span, this method is not appropriate. The IsRecording method returns if the span is recording or not. A read-only span value does not need to know if updates to it will be recorded or not. By definition, it cannot be updated so there is no point in communicating if an update is recorded.
• Removed the SpanSnapshot type from the go.opentelemetry.io/otel/sdk/trace package. The use of this type has been replaced with the use of the explicitly immutable ReadOnlySpan type. When a concrete representation of a read-only span is needed for testing, the newly added SpanStub in the go.opentelemetry.io/otel/sdk/trace/tracetest package should be used.
• Rename all *messageEvent* names to just be *Event*.

Resolves #1380

[codecov]

Codecov Report

Merging #1873 (187e4c0) into main (c99d5e9) will decrease coverage by 0.0%.
The diff coverage is 89.2%.

@@           Coverage Diff           @@
##            main   #1873     +/-   ##
=======================================
- Coverage   79.0%   79.0%   -0.1%     
=======================================
  Files        137     139      +2     
  Lines       7384    7477     +93     
=======================================
+ Hits        5837    5909     +72     
- Misses      1301    1322     +21     
  Partials     246     246             

Impacted Files	Coverage Δ
exporters/otlp/protocoldriver.go	100.0% <ø> (ø)
sdk/trace/simple_span_processor.go	88.0% <0.0%> (-0.5%)	⬇️
exporters/stdout/trace.go	69.5% <50.0%> (+4.3%)	⬆️
sdk/trace/snapshot.go	52.9% <52.9%> (ø)
sdk/trace/span.go	88.9% <71.0%> (-5.0%)	⬇️
sdk/trace/batch_span_processor.go	87.1% <75.0%> (ø)
sdk/trace/tracetest/span.go	96.7% <96.7%> (ø)
exporters/otlp/internal/otlptest/data.go	88.0% <100.0%> (+0.2%)	⬆️
exporters/otlp/internal/transform/span.go	98.1% <100.0%> (ø)
exporters/otlp/otlp.go	83.9% <100.0%> (ø)
... and 9 more

[jmacd]

I really read it. :)