Locust rounds response times into histogram buckets in StatsEntry._log_response_time. The rounding logic is not overridable, which means users who need different bucketing granularity have no clean way to change it. This PR extracts it into a module-level function so it can be replaced at runtime, the same way PERCENTILES_TO_CHART can be.

Our (MongoDB) use-case has us sometimes measuring things in the high microseconds/very low milliseconds range all the way up to the low seconds range. To get the needed range we report microseconds to Locust (I'm aware this is a horrible hack, sorry). For tests that need to cover both extremes we end up with too many buckets because the rounding cuts off at round(response_time, -3). Extracting the function gives us an avenue to fix that.

I expect usage to look something like:

import locust.stats
from math import floor, log10

def sigfig_bucket(response_time):
    """Bucket to 3 significant figures."""
    if response_time == 0:
        return 0
    return int(round(response_time, -int(floor(log10(abs(response_time)))) + 2))

locust.stats.bucket_response_time = sigfig_bucket

I annotated the argument as int | float since this is now a public interface and it is easier to later narrow the arg type than to broaden it without breaking user implementations. Happy to drop the float if you'd prefer to keep it strict. I'd like to discuss plumbing float through the stack at some point separately.

The docs update might be more than needed and I haven't tested how it renders. Happy to trim that.

Fixes #2005 (closed as stale). Related to #255.