Prevent duplicate "message" fields

[cloneable]

Bug Report

Description

The opentelemetry-rust people have started using tracing to log their internal errors, but decided to wrap tracing macro calls inside custom macros.

https://github.com/open-telemetry/opentelemetry-rust/blob/f46bccceab3a019fe4c2a46473dbef2c0d217702/opentelemetry/src/global/internal_logging.rs#L71-L78

Unfortunately, they also decided to set the "message" fields explicitly while their macros add an empty implicit "message" field.

https://github.com/open-telemetry/opentelemetry-rust/blob/f46bccceab3a019fe4c2a46473dbef2c0d217702/opentelemetry-sdk/src/trace/span_processor.rs#L381-L382

This tripped our custom subscriber. While we're fixing it and making it more robust, is there anything that can be done on tracing side to prevent duplicate "message" fields? Are FieldSets even meant to have only unique fields?

[cloneable]

Then again, if duplicate fields are allowed this would break the JSON logger.

tracing::info!(a = 1, a = 2, message = "bb", "aa");

Output of tracing_subscriber::fmt's JSON:

{"timestamp":"2025-01-16T10:05:22.290921Z","level":"INFO","fields":{"message":"aa","a":1,"a":2,"message":"bb"},"target":"tests"}