Graceful shutdown of Sink

[yarelm]

Hi.
In some cases, there is a graceful shutdown logic of the application (such as in Cloud Run), and I need a way to gracefully shutdown the Sink as well, so pending metrics will get sent upon closure and not lost.

[tam7t]

Hm, we could expand NewSink could accept a context and flush/shutdown on context cancellation (since #16 is already a breaking change)

Or could add a Flush() method to Sink struct so that the caller could do something likedefer s.Flush() or:

<-ctx.Done()
s.Flush()

(though flush itself should probably accept a context to add a deadline to the flush RPCs)

Or both

[yarelm]

Added #17

This one is not a breaking change and solves this issue.

[tam7t]

Added this to a tagged release: https://github.com/google/go-metrics-stackdriver/releases/tag/v0.4.0