cannot easily add labels through a logging filter

[nioncode]

We use python logging filters to automatically add labels to all our log entries without having to provide them individually for each log entry (similar to #750, but we need it with specific values for each request instead of for the entire app).

Usually, you add your filters through handler.addFilter(), but this results in our filter being called too late, because the CloudLoggingFilter already parsed the labels from the log record when itself got run and later additions to record.labels are ignored.
Interestingly, it works correctly if we use json_fields instead of labels, because the json_fields are evaluated only once the emit happens at which point all the filters have already run.

It would be great if this could be changed so that labels (and other properties) are also only evaluated once the entry is about to be emitted so that all logging filters have run already. In the meantime, this can be worked around by explicitly inserting our log filter as first filter in the filters list of a handler, but this involves messing around with internals of the logging library, which we'd like to avoid.

Is there a reason why the fields are added with a log filter anyways and not during explicitly during emit?

Environment details

• OS type and version: Ubuntu 24.04
• Python version: 3.9.18
• pip version: 23.0.1
• google-cloud-logging version: 3.11.2

Code example

def add_request_id_filter(record):
    if not hasattr(record, "labels"):
        record.labels = {}
    record.labels["request_id"] = g.request_id
    return True

# Does not work.
root_logger = logging.getLogger()
for handler in root_logger.handlers:
    handler.addFilter(add_request_id_filter)

# This works instead
root_logger = logging.getLogger()
for handler in root_logger.handlers:
    handler.filters.insert(0, add_request_id_filter)