[[output.nats]] Bytes limits on output size

Use Case

In NATS, messages have a max size (1Mb by default).

As telegraf limits the size of the batch in number of messages (instead of bytes), it's not very useful in the case when one message can contain multiple lines of lineprotocol output, making messages' actual length very wide.

Expected behavior

It is possible to limit the messages sent to NATS to avoid triggering the message limit size.

This feature is already implemented on other output plugins such as cloud_pubsub, event_hubs, file, graylog and influxdb (thanks @Hipska for the research).

Actual behavior

Given a big enough metric_batch_size value, NATS max message size error is triggered when the serialized message exceeds the max message size (1Mb by default).

Additional info

No response

Start with the output.nats plugin and compare its batching behavior with the cloud_pubsub, event_hubs, file, graylog, and influxdb output plugins, which already support size limits. Trace how serialized messages are formed and identify the existing tests for these plugins. Done means NATS messages can be limited by bytes as well as metric_batch_size, preventing the reported maximum-size error.