Multiprocess mode support for 'remove'

In multiprocess mode calling 'remove' on a metric/label doesn't actually remove it from the metrics output.

Also, because of the disconnect between the Metric classes's remove and MultiProcessValue, the following snippet will use more memory every iteration as the MultiProcessValue.values list keeps adding copies.

import os
os.environ['PROMETHEUS_MULTIPROC_DIR'] = '/tmp'

from prometheus_client import Counter

c = Counter('a', 'a', ['a'])
while True:
    c.labels('hi').inc()
    c.remove('hi')

Perhaps 'remove' should be disabled in multiprocess mode, or at least something added to the multiprocess documentation?

首先在 multiprocess 模式下复现提供的循环，然后结合 MultiProcessValue 和 multiprocess 文档检查 Metric 类的 remove 行为。确定删除应如何影响已发出的指标和不断增长的 values 列表；完成的标准是：所选行为已实现并由回归测试覆盖，或者该限制已得到明确记录。