Reduce default concurrent requests to 1

[npepinpe]

By default, the exporter uses an idempotent producer with "infinite" retries and delivery time out. However, it also configures a max of 3 concurrent requests, which may result (due to retries) in some batches being delivered out of order. To avoid this, the default should lower it to a single request, which anyway should be fine for most use cases.

It may not even be advisable to expose the configuration option as we currently do, and leave it up to advanced users to configure it via the free-form producer properties if they really want to.