[Feature]: Add support for custom error handlers + make `SdkLogRecord::new` public again

[mdecimus]

Related Problems?

Hi,

We just migrated Stalwart from opentelemetry 0.25 to 0.29 and noticed two issues that I briefly explained in #2260:

• We use our custom logging backend rather than tracing and global::set_error_handler has been removed.
• We also use run own own span, log and metrics export task and the Default trait has been removed from SdkLogRecord. In order to be able to generate a SdkLogRecord we now need to do SdkLoggerProvider::builder().build().logger("stalwart").create_log_record() which is not awful but it would be much nicer if SdkLogRecord::new was made public or the Default trait was added.

Describe the solution you'd like:

I propose to:

• Re-implement global::set_error_handler (or similar) and place it behind a Cargo feature, which will be disabled by default. I agree that most users will use tracing.
• Make SdkLogRecord::new public or add the Default trait to SdkLogRecord.

I'm not familiar with the internals of this crate but if you prefer I can submit a PR for these.

Considered Alternatives

No response

Additional Context

No response

[cijothomas]

Thanks for opening this. Yes it is absolutely acceptable to make the internal logging support a callback instead of just the default way of tracing. If you have bandwidth, please open a PR (draft is fine) so we can discuss the direction/overall-design.