Support sending a random routing key for each message in kafka output

[danielnelson]

Feature Request

Proposal:

Kafka requires at least one partition per concurrent consumer, and without a properly set routing key (aka partition key) only one partition will be written too. There is some information suggesting that the partition written to will be changed after 10m, but this would be too long of a windows for most uses.

While sometimes it is possible to use a common tag such as host, and in many respects this is ideal as it maintains ordering of each series, this tag is not always set.

Add a new parameter to the Kafka output plugin to add a random routing key to each message.

Current behavior:

Messages can be sent without a routing key or using the routing_tag option to choose a tag whose value is used as the routing key.

Desired behavior:

Similar to the amqp output, add a new routing_key option to set a static key, or if set to "random" then generate a random key.

[[outputs.kafka]]
  routing_key = "random"

Use case:

Without a common tag between all metrics this is required for spreading writes to multiple partitions.