[KafkaJS] Support single span for bulk producer message send

[SeanReece]

Is your feature request related to a problem? Please describe

When using producer.send or producer.sendBatch to send multiple messages, a span is created for every message sent. This creates a lot of noise when bulk sending messages (We frequently send 1000s of events in a single bulk call), and this is not how kafkaJS produces messages, which seems to be only split by topic/partition/broker.

See here for message sending logic in KafkaJS which is used for send and sendBatch: https://github.com/tulios/kafkajs/blob/master/src/producer/sendMessages.js#L17

Example

Describe the solution you'd like to see

I think this could either be configurable to only add spans to each call to producer.send (maybe include a messageCount attribute)

Example

instrumentations: [
    new KafkaJsInstrumentation({ groupProducerSend: true }),
],

OR the logic could be changed to wrap broker.produce (is this possible?) which would more closely represent how KafkaJS interacts with the broker.

Additional context

const producer = kafka.producer()

await producer.connect()
await producer.send({
    topic: 'topic-name',
    messages: [
        { key: 'key1', value: 'hello world' },
        { key: 'key2', value: 'hey hey!' }
    ],
})

The above snippet would create 2 spans currently, even though there is only 1 call to send and KafkaJS will send these to the broker in a single request.

Thanks!