Add a Shutdown() method to TracerProvider

[fbogsany]

See open-telemetry/opentelemetry-specification#1074

[fbogsany]

Ironically, it turns out we've had this "forever":

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

Lines 44 to 61 in dd805ec

	# Attempts to stop all the activity for this {Tracer}. Calls
	# SpanProcessor#shutdown for all registered SpanProcessors.
	#
	# This operation may block until all the Spans are processed. Must be
	# called before turning off the main application to ensure all data are
	# processed and exported.
	#
	# After this is called all the newly created {Span}s will be no-op.
	def shutdown
	@mutex.synchronize do
	if @stopped
	OpenTelemetry.logger.warn('calling Tracer#shutdown multiple times.')
	return
	end
	@active_span_processor.shutdown
	@stopped = true
	end
	end

It looks like we need minor documentation tweaks - I think this was moved from Tracer to TracerProvider at some point.