A sampling window that recorded nothing leaves the histogram empty, and an
empty IntervalHistogram reports its mean as NaN. Its percentiles return a floor
instead, which is why only the mean was affected and why this went unnoticed.
NaN then survives every arithmetic step in the sampler without complaint and
becomes visible only at the edge, where JSON.stringify renders it as null. So
/api/status served "mean_ms": null while nothing raised an error anywhere, and
any consumer of that gauge read null instead of a number.
Non-finite readings now report 0, which is the honest value: no samples means
no measured delay. Applied to every field so a later change to the histogram
source cannot reintroduce this one field at a time.
Found by CI rather than locally, because an idle window is far likelier on a
loaded runner with several test servers in flight. The failure was real; the
new tests establish the NaN premise and the null serialisation directly rather
than relying on that timing to reproduce.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Uaeo9MvzKoyXuN6ZsbhtkL
