Use setInterval to upload metrics to Stackdriver periodically

[rueian]

Hi,

I made some changes to let stackdriver monitoring exporter upload metrics periodically, which are related to issue #152. Including:

1. Use setInterval() to schedule metrics uploading.
2. Add the close() method which can clear the setInterval timer.
3. Change the behavior of .onRecord() and related test cases, because metrics will not be uploaded when .onRecord() is called.
4. Make sure .catch() blocks in some test cases are executed by finishing them with mocha done()
5. Map all non AggregationType.LAST_VALUE to stackdriver MetricKind.CUMULATIVE.

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CLAs look good, thanks!

[kjin]

Suggested change

	const timeSeries = [] as TimeSeries[];
	const timeSeries: TimeSeries[] = [];

[kjin]

@isaikevych please merge at your leisure.

@rueian thanks for the contribution!

[isaikevych]

Thank you

[isaikevych]

Fixes #152