feat: add spans to Trace::ExportError

[fbogsany]

This adds a spans attribute to SDK::Trace::ExportError, containing the array of spans that could not be exported. It refactors the BSP a little to ensure an array of Span rather than (sometimes) an array of SpanData is wrapped by the exception.

The intent is to allow a pluggable error handler to recognize the error type, retrieve the array of spans, and (for example) report more detailed information (such as the trace IDs of the spans that are dropped).

Example:

OpenTelemetry.error_handler = ->(exception: nil, message: nil) do
  case exception
  when OpenTelemetry::SDK::Trace::ExportError
    logger.error("OpenTelemetry error: #{exception.message} from trace IDs: #{exception.spans.collect {|span| span.context.trace_id }.uniq.join(', ')}")
  else
    logger.error("OpenTelemetry error: #{[message, exception&.message].compact.join(' - ')}")
  end
end

[fbogsany]

It'd be useful to provide documentation for the spans arg to initialize and the type of the spans attribute.

[plantfansam]

Proving to myself that you can do instance vars + attr_reader on an exception

class FooError < StandardError
  attr_reader :str
  def initialize(str)
    super("str")
    @str = str
  end
end

begin
  raise FooError.new("hi")
rescue FooError => e
  pp "in rescue block"
  pp e.str
end

ruby foo.rb
"in rescue block"
"hi"

[plantfansam]

✅ calling to_span_data here instead of elsewhere in bsp
✅ no longer mutating span_array so that we can pass an array of Spans to report_result