Measurement consumer simulator only works with sampling rate = 1.0

[ple13]

Describe the bug
The measurement consumer simulator gets the measurement results from the MPC systems, then compares them with the ground truth. The ground truth is computed based on the sampled vids instead of the full set. When the sampling rate is not 1.0, the computed ground truth is incorrect.

Steps to reproduce

1. Update the measurement specs here or here with sampling width interval = 0.27.
2. Run the accuracy tests and read the log. Search for "round=" to see the discrepancy between estimated reach and actual reach.

Component(s) affected
Integration and accuracy tests.

Version
commit e34610d

Environment
Local testing.

Additional context
Sampling Interval Width = 0.27
Aug 08, 2024 4:27:26 PM org.wfanet.measurement.integration.common.InProcessReachMeasurementAccuracyTest$reach-only hmss results should be accurate with respect to the variance$1 invokeSuspend
INFO: round=1, actual_result=9559, expected_result=1605, interval=(-1412.78, 20530.78), accurate=true

Sampling Interval Width = 1.0
Aug 08, 2024 6:45:06 PM org.wfanet.measurement.integration.common.InProcessReachMeasurementAccuracyTest$reach-only hmss results should be accurate with respect to the variance$1 invokeSuspend
INFO: round=1, actual_result=5446, expected_result=6000, interval=(2484.72, 8407.28), accurate=true