Byte-exactness tests stay green if the compressed range seek regresses into
decoding from byte zero: the returned bytes are still correct and only the
read amplification explodes. Assert the cost side as well.
The observation reuses rustfs_io_get_object_shard_read_observed_bytes_total,
already emitted per shard read by the erasure layer, so no production code is
instrumented. The OTLP collector learns to accumulate a second counter, keyed
by its path/role/outcome labels rather than by data-point position, which is
not stable across exports.
Two failure modes the assertions guard against:
- With RUSTFS_OBS_METER_INTERVAL=1, treating one unchanged sample as settled
measures a delta of zero, because the range read's counter has not been
exported yet. Settling now requires several consecutive equal samples.
- An upper bound alone passes vacuously on a zero delta, so a lower bound
turns "measured nothing" into a failure instead of a green run.
Refs rustfs/rustfs#5957, backlog#1848.
