Add a convenient method to DistributionMetricSubject

Currently to assert that a given Metric<Distribution> as a certain distribution for some labels, the caller needs to manually create an ImmutableDistribution and pass it to #hasValueForLabels method. With this change, an ImmutableSet of data points can be passed to #hasDataSetForLabels method.

Also switched to use expectThrow backport from JUnit 4.13.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=173544521

java/google/registry/monitoring/metrics/contrib/AbstractMetricSubject.java

@@ -107,7 +107,7 @@ abstract class AbstractMetricSubject<T, S extends AbstractMetricSubject<T, S>>
     }
     if (!metricPoint.value().equals(value)) {
       failWithBadResults(
-          String.format("has a value of %s for labels", value),
+          String.format("has a value of %s for labels", getMessageRepresentation(value)),
           Joiner.on(':').join(labels),
           "has a value of",
           getMessageRepresentation(metricPoint.value()));