Support specifying batch limit

[mluggy]

On high-velocity metrics, you get:
UnhandledPromiseRejectionWarning: Error: Field timeSeries had an invalid value: A maximum of 200 TimeSeries can be written in a single request.

Apparently Stackdriver backend has the limit that each request cannot batch more than 200 time series so exporter needs to do some processing. For example https://github.com/census-instrumentation/opencensus-java/blob/9b2ac2bdc9565517155a9efb1b8581d1a91329dc/exporters/stats/stackdriver/src/main/java/io/opencensus/exporter/stats/stackdriver/CreateTimeSeriesExporter.java#L76-L77.

Using 0.0.16.

[mayurkale22]

@mluggy Thanks for reporting this! I managed to repro the issue. I have opened the PR #644 to address this, Please take a look when you have a time.

[mluggy]

looks awesome.