No quantiles in multiprocess mode

[faevourite]

Hi there. Thanks for this library!

It seems to work well in normal single-process mode:

# HELP asdfasdf asdfasdf
# TYPE asdfasdf summary
asdfasdf{quantile="0.5"} 1.1203176259996326
asdfasdf{quantile="0.9"} 1.1203176259996326
asdfasdf{quantile="0.99"} 1.1203176259996326
asdfasdf_count 1.0
asdfasdf_sum 1.1203176259996326

However, when running in multi-process mode, quantiles don't get added to metrics:

# HELP asdfasdf asdfasdf
# TYPE asdfasdf summary
asdfasdf_count 1.0
asdfasdf_sum 1.3480212929998743

Versions:

prometheus-client==0.23.1
prometheus-fastapi-instrumentator==7.1.0
prometheus-summary==0.1.6