Assert over metrics sent to Graphite

It would be great if we could have a way to assert over metrics sent by the `GraphiteSink` to an actual Graphite server.

At the moment we have similar tests for the `InMemoryMetrics` sink, which give a good level of confidence (that's why I've set this to Minor), but we might be able to stretch things even further by capturing metrics sent by the graphite reporter in the `GraphiteSink` using an in memory implementation of a simple Graphite server.

This could also be useful for other Beam components once they start reporting to Graphite.

Imported from Jira [BEAM-1215](https://issues.apache.org/jira/browse/BEAM-1215). Original Jira may contain additional context.
Reported by: staslev.

Start by locating GraphiteSink and the existing InMemoryMetrics tests. Then trace how GraphiteSink sends metrics and identify the test entry points for an in-memory Graphite server. Done means tests can capture and assert the metrics sent by GraphiteSink, with coverage comparable to the existing InMemoryMetrics tests.

Written by the indexing model from the issue text.