Allow injecting custom fields into logger from prefect.context

[jacques-]

Current behavior

In our system we pass trace_id from event source through to all systems to close so we can track end to end and correlate with performance traces and metrics. We put the trace_id into the prefect.context and use that to enrich traces and metrics, however the final part of the puzzle is to add to logs as well.

I spoke to Dylan on slack, and he very kindly pointed me at this: https://github.com/PrefectHQ/prefect/blob/master/src/prefect/utilities/logging.py#L170 which does exactly what I need. However, so far as I can tell only a static list of context attributes are injected by default (from PREFECT_LOG_RECORD_ATTRIBUTES).

I've found a way around this but it's super hacky:

prefect.utilities.prefect.utilities.logging.PREFECT_LOG_RECORD_ATTRIBUTES = (
            PREFECT_LOG_RECORD_ATTRIBUTES + ("trace_id",)
        )

Proposed behavior

Would be great if this Tuple was a configurable list, possibly in the context object.

I was thinking of a small patch to _log_record_context_injector that would look for a prefect.context.additionally_injected_log_attributes list and if found, include those values. If this seems like something you would merge I'd be happy to do a PR?

Example

If this list was configurable it would allow setting a custom logging format that includes these injected fields so that we have our trace_id in each log message.