Reach value is not multiple of the reciprocal of vidSamplingRate

[riemanli]

Describe the bug
A differential private reach value is calculated by

reach_dp = (reach + noise) / sampling_rate

Since the noise used in the system is discrete Laplace and so is integer, the differential private reach values outputted from the measurement system should always be multiple of the reciprocal of the sampling rate. However, based on the reach values gotten from the measurement system, it's not for both direct measurements and multi-publisher measurements.

Steps to reproduce

1. Use the MeasurementSystem tool to submit create measurement request for reach with either single publisher or multi-publisher and argument --vid-sampling-width == 0.01. Ex:

bazel-bin/src/main/kotlin/org/wfanet/measurement/api/v2alpha/tools/MeasurementSystem \
--tls-cert-file=src/main/k8s/testing/secretfiles/mc_tls.pem \
--tls-key-file=src/main/k8s/testing/secretfiles/mc_tls.key \
--cert-collection-file=src/main/k8s/testing/secretfiles/kingdom_root.pem \
--kingdom-public-api-target=v2alpha.kingdom.dev.halo-cmm.org:8443 \
measurements \
--api-key=<API_KEY> \
create \
--measurement-consumer=measurementConsumers/VCTqwV_vFXw \
--reach-and-frequency \
--reach-privacy-epsilon=0.0041 \
--reach-privacy-delta=0.000000000001 \
--frequency-privacy-epsilon=0.0001 \
--frequency-privacy-delta=0.000000000001 \
--reach-max-frequency=1 \
--vid-sampling-start=0.0 \
--vid-sampling-width=0.01 \
--private-key-der-file=src/main/k8s/testing/secretfiles/mc_cs_private.der \
--measurement-ref-id=502 \
--data-provider=dataProviders/FeQ5FqAQ5_0 \
--event-group=dataProviders/FeQ5FqAQ5_0/eventGroups/YSMZ7X_D5Iw \
--event-start-time=2021-03-05T00:00:00.01Z \
--event-end-time=2021-03-05T23:59:59.99Z

2. Check if the reach value is multiple of 100.

Component(s) affected
Kingdom, testing simulators

Version
6184fb9

Environment
QA env

Additional context
We can confirm the direct measurement is not computed correctly as shown in EdpSimulator.kt#L835. Further investigation is needed for the multi-publisher case.