Attribute limits should discard from the end

[fbogsany]

Description of the bug

See #1533 (comment) for background. open-telemetry/opentelemetry-specification#1130 added span limits to the spec. A key requirement is:

• set a limit of unique attribute keys such that:
• for each unique attribute key, addition of which would result in exceeding
  the limit, SDK MUST discard that key/value pair.

Our implementation of span limits instead drops attributes added earlier when the attribute count limit is exceeded:

opentelemetry-ruby/sdk/lib/opentelemetry/sdk/trace/span.rb

Lines 347 to 354 in afc7bc5

	def trim_span_attributes(attrs)
	return if attrs.nil?
	excess = attrs.size - @span_limits.attribute_count_limit
	excess.times { attrs.shift } if excess.positive?
	truncate_attribute_values(attrs, @span_limits.attribute_length_limit)
	nil
	end

E.g. if ENV['OTEL_ATTRIBUTE_COUNT_LIMIT'] == '2' then the following code should produce a span with attributes {"a" => 1, "b" => 2} but instead it contains {"b" => 2, "c" => 3}:

tracer.in_span('bad limit', attributes: {"a" => 1, "b" => 2}) do |span|
  span["c"] = 3
end

Share a simplified reproduction if possible

ENV['OTEL_ATTRIBUTE_COUNT_LIMIT'] = '2'

require 'bundler/inline'

gemfile(true) do
  source 'https://rubygems.org'

  gem 'opentelemetry-api'
  gem 'opentelemetry-sdk'
end

require 'opentelemetry-api'
require 'opentelemetry-sdk'

span_processor = OpenTelemetry::SDK::Trace::Export::SimpleSpanProcessor.new(
  OpenTelemetry::SDK::Trace::Export::ConsoleSpanExporter.new
)

OpenTelemetry::SDK.configure do |c|
  c.add_span_processor(span_processor)
end

SimpleTracer = OpenTelemetry.tracer_provider.tracer('Bug Report')

SimpleTracer.in_span('bad limit', attributes: {"a" => 1, "b" => 2}) do |span|
  span["c"] = 3
end

OpenTelemetry.tracer_provider.shutdown