chore: Update README and conditionally flush metrics

Author: joshwestbrook

instrumentation/aws_lambda/lib/opentelemetry/instrumentation/aws_lambda/wrap.rb

@@ -60,7 +60,7 @@ def wrap_lambda(event:, context:, handler:, flush_timeout: DEFAULT_FLUSH_TIMEOUT
           end
 
           OpenTelemetry.tracer_provider.force_flush(timeout: flush_timeout)
-          OpenTelemetry.meter_provider.force_flush(timeout: flush_timeout)
+          OpenTelemetry.meter_provider.force_flush(timeout: flush_timeout) if OpenTelemetry.respond_to?(:meter_provider)
 
           raise original_handler_error if original_handler_error