'Adds the counts of another Histogram into this instance.
`other` must have the same buckets as this instance. The counts
from `other` are added to the counts for this instance.
Args:
other: Another Histogram instance with the same buckets as this instance.
Raises:
MergeStatisticsException: If `other` is not a Histogram or the buckets
are not the same.'

def _merge_from(self, other):

    if (not isinstance(other, Histogram)):
        raise MergeStatisticsException(('Cannot    merge    %s    into    Histogram' % other.__class__.__name__))
    if (self.buckets != other.buckets):
        raise MergeStatisticsException(('Histogram    buckets    do    not    match.    Expected    %s,    got    %s' % (self.buckets, other.buckets)))
    for (bucket_lower, count) in other.counters.items():
        self.counters[bucket_lower] += count
