use new metricdata/exemplar

[aviaoh]

due to census-instrumentation/opencensus-go#1067, go.opencensus.io/exemplar doesn't exist any more.

this PR uses the new package

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[aviaoh]

@songy23 @rghetia @bogdandrutu @rakyll

[aviaoh]

I signed it! (cla)

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[songy23]

Thanks for fixing this!