Document inputs who (can) produce tracking metrics

[Hipska]

Use Case

To have a better view which inputs support this, it should be documented on their README page.

Subtasks

• Identify which plugins are actually able to produce tracking metrics
• Modify README for plugins indicating they produce tracking metrics

Plugins with tracking metrics

• inputs.amqp_consumer
• inputs.cloud_pubsub
• inputs.directory_monitor (Only for keeping track of number of undelivered metrics)
• inputs.eventhub_consumer (Only for keeping track of number of undelivered metrics, not sure about this)
• intputs.influxdb_v2_listener (Only for keeping track of number of undelivered metrics)
• inputs.kafka_consumer
• inputs.kinesis_consumer
• inputs.mqtt_consumer
• inupts.nats_consumer (Only for keeping track of number of undelivered metrics)
• inputs.nsq_consumer
• inputs.tail (Only for keeping track of number of undelivered metrics)