Add metrics in the exporters and span processors

[fbogsany]

At least the following metrics are useful.

In exporters:

• export.failure - counts failed export requests
• export.success - counts successful export requests
• export.spans - counts spans successfully exported

In BatchSpanProcessor and SimpleSpanProcessor:

• dropped_spans - counts spans permanently dropped

Alternatively, hooks could be provided in the span processors and exporters to allow reporting these metrics. E.g. some may wish to log them instead, and indeed we do this currently in an ad-hoc fashion:

opentelemetry-ruby/sdk/lib/opentelemetry/sdk/trace/export/batch_span_processor.rb

Lines 127 to 129 in 6f6127e

	def report_result(result_code, batch)
	OpenTelemetry.logger.error("Unable to export #{batch.size} spans") unless result_code == SUCCESS
	end

and

opentelemetry-ruby/sdk/lib/opentelemetry/sdk/trace/export/simple_span_processor.rb

Lines 51 to 53 in 6f6127e

	rescue => e # rubocop:disable Style/RescueStandardError
	OpenTelemetry.logger.error("unexpected error in span.on_finish - #{e}")
	end

[fbogsany]

DataDog has a good example of common health metrics that we could also consider for inspiration.

[fbogsany]

This has been implemented for the BatchSpanProcessor and the OTLP Exporter:

• otel.otlp_exporter.request_duration - duration of export request with status label (response code or unknown)
• otel.otlp_exporter.failure - counts failed export requests with reason label (response code or timeout)
• otel.bsp.buffer_utilization - observes buffer utilization
• otel.bsp.export.success - counts successful export requests
• otel.bsp.exported_spans - counts spans successfully exported
• otel.bsp.export.failure - counts failed export requests
• otel.bsp.dropped_spans - counts spans permanently dropped with reason label (export-failure, terminating, buffer-full)

Closing this.